diff --git a/lib/rackstash.rb b/lib/rackstash.rb
index 8715425..90544bd 100644
--- a/lib/rackstash.rb
+++ b/lib/rackstash.rb
@@ -173,6 +173,7 @@ require 'rackstash/adapter/logger'
 require 'rackstash/adapter/io'
 require 'rackstash/adapter/null'
 
+require 'rackstash/encoder/gelf'
 require 'rackstash/encoder/hash'
 require 'rackstash/encoder/json'
 require 'rackstash/encoder/lograge'
diff --git a/lib/rackstash/encoder/gelf.rb b/lib/rackstash/encoder/gelf.rb
new file mode 100644
index 0000000..80e2968
--- /dev/null
+++ b/lib/rackstash/encoder/gelf.rb
@@ -0,0 +1,206 @@
+# frozen_string_literal: true
+#
+# Copyright 2017 Holger Just
+#
+# This software may be modified and distributed under the terms
+# of the MIT license. See the LICENSE.txt file for details.
+
+require 'date'
+require 'socket'
+require 'time'
+
+require 'rackstash/encoder'
+require 'rackstash/encoder/helper/fields_map'
+require 'rackstash/encoder/helper/message'
+require 'rackstash/helpers/utf8'
+
+module Rackstash
+  module Encoder
+    # The Graylog Extended Log Format (GELF) is the native format used by
+    # [Graylog](https://www.graylog.org). By formatting Rackstash's log events
+    # as GELF, you can directly send thm to a Graylog server for storage and
+    # further processing.
+    #
+    # GELF is based in JSON with some additional restrictions. Please see
+    # [the specification](http://docs.graylog.org/en/2.4/pages/gelf.html) for
+    # details. The GELF encoder returns the log event as a JSON-encoded `String`
+    # without any literal newline characters.
+    #
+    # To send Rackstash log events to a Graylog server, you can use the
+    # {Adapter::GELF} adapter to send the formatted GELF payload to your Graylog
+    # server using any of the supported transport protocols.
+    #
+    # @see http://docs.graylog.org/en/2.4/pages/gelf.html
+    class GELF
+      include Rackstash::Encoder::Helper::FieldsMap
+      include Rackstash::Encoder::Helper::Message
+      include Rackstash::Helpers::UTF8
+
+      # The default mapping of GELF fields (the keys) to fields in the final
+      # Rackstash event hash (the value). You can overwrite this mapping by
+      # setting the `fields` parameter in {#initialize}.
+      DEFAULT_FIELDS = {
+        host: nil, # local hostname by default
+        level: nil, # highest severity of an event message mapped to a syslog level
+        short_message: FIELD_MESSAGE,
+        # omitted by default
+        long_message: nil,
+
+        # The event's timestamp
+        timestamp: FIELD_TIMESTAMP
+      }.freeze
+
+      # Mapping of Rackstash log severities to the syslog levels used by GELF
+      GELF_LEVELS = {
+        DEBUG => 7,  # Debug
+        INFO => 6,   # Informational
+        WARN => 5,   # Notice
+        ERROR => 4,  # Warning
+        FATAL => 3,  # Error
+        UNKNOWN => 1 # Alert – shouldn't be used
+      }.freeze
+
+      # @param fields [Hash<Symbol => String, nil>] a mapping of standard fields
+      #   in the emitted GELF message (the Hash keys) to their respective source
+      #   fields in the passed Rackstash event (the values). By default, we use
+      #   the {DEFAULT_FIELDS} mapping which can selectively be overwritten with
+      #   this `fields` argument. All fields in the event Hash which are not
+      #   mapped to one of the main GELF fields will be added as additional
+      #   GELF fields. If the mapped value is `nil`, we do not include the field
+      #   or set it with a default value.
+      # @param default_severity [Integer] The default log severity. One of the
+      #   {SEVERITIES} constants. If the `level` field of the generated GELF
+      #   message is not overwritten with another field and we can not determine
+      #   a maximum severity from the event's messages, we emit the syslog level
+      #   matching this severity in the `level` field of the generated message.
+      def initialize(fields: {}, default_severity: UNKNOWN)
+        set_fields_mapping(fields, DEFAULT_FIELDS)
+        @default_severity = Rackstash.severity(default_severity)
+      end
+
+      # Encode the passed event Hash as a JSON string following the GELF
+      # specification.
+      #
+      # @param event [Hash] a log event as produced by the {Flow}
+      # @return [String] the GELF-formatted event as a single-line JSON string
+      def encode(event)
+        gelf = {}
+
+        # > GELF spec version – "1.1"; MUST be set by client library.
+        gelf['version'] = '1.1'.freeze
+
+        # > the name of the host, source or application that sent this message;
+        # > MUST be set by client library.
+        host = extract_field(:host, event) { Socket.gethostname }
+        gelf['host'] = utf8_encode(host)
+
+        # > Seconds since UNIX epoch with optional decimal places for
+        # > milliseconds; SHOULD be set by client library. Will be set to the
+        # > current timestamp (now) by the server if absent.
+        timestamp = extract_field(:timestamp, event)
+        gelf['timestamp'] = gelf_timestamp(timestamp)
+
+        # > the level equal to the standard syslog levels; optional, default is
+        # > `1` (ALERT)
+        # The default value of 1 corresponds to {UNKNOWN} in Rackstash.
+        level = extract_field(:level, event) {
+          GELF_LEVELS.fetch(max_message_severity(event)) {
+            GELF_LEVELS[@default_severity]
+          }
+        }
+        gelf['level'] = Integer(level)
+
+        # Since we need the raw messages to find the GELF level above, we only
+        # now normalize the message array to a simple String here
+        normalize_message(event)
+
+        # > a short descriptive message; MUST be set by client library.
+        short_message = extract_field(:short_message, event) { EMPTY_STRING }
+        gelf['short_message'] = utf8_encode(short_message)
+
+        # > a long message that can i.e. contain a backtrace; optional.
+        #
+        # Since the field is optional, we only write this field if there is a
+        # value in our event hash
+        full_message = extract_field(:full_message, event)
+        gelf['full_message'] = utf8_encode(full_message) if full_message
+
+        gelf.merge! additional_fields(event)
+
+        ::JSON.dump(gelf)
+      end
+
+      private
+
+      def gelf_timestamp(timestamp)
+        time = case timestamp
+        when Time, DateTime, Date
+          timestamp.to_time
+        when String
+          Time.iso8601(timestamp) rescue Time.now.utc
+        when Integer, Float
+          timestamp
+        else
+          Time.now.utc
+        end
+
+        time.to_f
+      end
+
+      def max_message_severity(event)
+        messages = event[FIELD_MESSAGE]
+        return @default_severity unless messages.is_a?(Array)
+
+        max_severity = nil
+        messages.each do |message|
+          next unless message.respond_to?(:severity)
+          severity = message.severity
+
+          next if severity >= UNKNOWN
+          next if max_severity && severity < max_severity
+
+          max_severity = severity
+        end
+
+        max_severity || @default_severity
+      end
+
+      def additional_fields(event)
+        additional_fields = {}
+
+        event.each_pair do |key, value|
+          # "_id" is reserved, so use "__id"
+          key = '_id'.freeze if key == 'id'.freeze
+          add_additional_field(additional_fields, "_#{key}", value)
+        end
+        additional_fields
+      end
+
+      def add_additional_field(result, key, value)
+        case value
+        when ::Hash
+          value.each_pair do |hash_key, hash_value|
+            add_additional_field(result, "#{key}.#{hash_key}", hash_value)
+          end
+        when ::Array
+          value.each_with_index do |array_value, index|
+            add_additional_field(result, "#{key}.#{index}", array_value)
+          end
+        when ::Time, ::DateTime
+          value = value.to_time.getutc
+          result[sanitize(key)] = value.iso8601(ISO8601_PRECISION)
+        when ::Date
+          result[sanitize(key)] = value.iso8601
+        else
+          result[sanitize(key)] = value
+        end
+      end
+
+      def sanitize(key)
+        key.gsub(/[^\w\.\-]/, '_'.freeze)
+      end
+    end
+
+    register GELF, :gelf
+  end
+end
diff --git a/lib/rackstash/encoder/helper/fields_map.rb b/lib/rackstash/encoder/helper/fields_map.rb
new file mode 100644
index 0000000..cd22404
--- /dev/null
+++ b/lib/rackstash/encoder/helper/fields_map.rb
@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+#
+# Copyright 2018 Holger Just
+#
+# This software may be modified and distributed under the terms
+# of the MIT license. See the LICENSE.txt file for details.
+
+require 'rackstash/helpers/utf8'
+
+module Rackstash
+  module Encoder
+    module Helper
+      # Some useful helper methods for {Rackstash::Encoder}s which help in
+      # normalizing and handling the message list in the event Hash.
+      module FieldsMap
+        include Rackstash::Helpers::UTF8
+
+        private
+
+        def set_fields_mapping(fields, default = {})
+          @fields_map = default.dup
+          Hash(fields).each_pair do |key, value|
+            @fields_map[key.to_sym] = utf8_encode(value)
+          end
+        end
+
+        def extract_field(name, event)
+          field_name = @fields_map[name]
+
+          field = event.delete(field_name) if field_name
+          field = yield(field_name) if field.nil? && block_given?
+          field
+        end
+      end
+    end
+  end
+end
diff --git a/spec/rackstash/encoder/gelf_spec.rb b/spec/rackstash/encoder/gelf_spec.rb
new file mode 100644
index 0000000..435b170
--- /dev/null
+++ b/spec/rackstash/encoder/gelf_spec.rb
@@ -0,0 +1,246 @@
+# frozen_string_literal: true
+#
+# Copyright 2018 Holger Just
+#
+# This software may be modified and distributed under the terms
+# of the MIT license. See the LICENSE.txt file for details.
+
+require 'spec_helper'
+
+require 'rackstash/encoder/gelf'
+
+RSpec.describe Rackstash::Encoder::GELF do
+  let(:encoder_args) { {} }
+  let(:encoder) { described_class.new(**encoder_args) }
+
+  describe 'version field' do
+    it 'adds a fixed version' do
+      expect(encoder.encode({})).to include '"version":"1.1"'
+    end
+  end
+
+  describe 'host field' do
+    it 'adds the current host by default' do
+      allow(Socket).to receive(:gethostname).and_return('foo')
+      expect(encoder.encode({})).to include '"host":"foo"'
+    end
+
+    it 'uses the configured host field' do
+      encoder_args[:fields] = { host: 'host_field' }
+
+      expect(Socket).not_to receive(:gethostname)
+      expect(encoder.encode('host_field' => 'foo')).to include '"host":"foo"'
+      expect(encoder.encode('host_field' => 'foo')).not_to include 'host_field'
+    end
+
+    it 'adds the current host if the host field is missing' do
+      encoder_args[:fields] = { host: 'host_field' }
+
+      allow(Socket).to receive(:gethostname).and_return('localhorst.example.com')
+      expect(encoder.encode({})).to include '"host":"localhorst.example.com"'
+    end
+  end
+
+  describe 'timestamp field' do
+    it 'formats the event timestamp' do
+      event = { '@timestamp' => DateTime.new(2016, 10, 17, 16, 37, 0, '+03:00') }
+      expect(encoder.encode(event)).to include '"timestamp":1476711420.0'
+      expect(encoder.encode(event)).not_to include '@timestamp'
+    end
+
+    it 'formats a DateTime' do
+      event = { '@timestamp' => DateTime.new(2016, 10, 17, 16, 37, 0, '+03:00') }
+      expect(encoder.encode(event)).to include '"timestamp":1476711420.0'
+    end
+
+    it 'formats a Date' do
+      event = { '@timestamp' => Date.new(2016, 10, 17) }
+      expect(encoder.encode(event)).to include '"timestamp":1476655200.0'
+    end
+
+    it 'formats a String' do
+      event = { '@timestamp' => '2016-10-17T16:37:00+03:00' }
+      expect(encoder.encode(event)).to include '"timestamp":1476711420.0'
+    end
+
+    it 'formats an Integer' do
+      event = { '@timestamp' => 1476711420 }
+      expect(encoder.encode(event)).to include '"timestamp":1476711420.0'
+    end
+
+
+    it 'adds the current timestamp by default' do
+      allow(Time).to receive(:now)
+        .and_return Time.new(2017, 12, 17, 16, 37, 0, '+03:00')
+      expect(encoder.encode({})).to include '"timestamp":1513517820.0'
+    end
+  end
+
+  describe 'level field' do
+    it 'uses the configured level field' do
+      encoder_args[:fields] = { level: 'level_field' }
+
+      messages = [instance_double('Rackstash::Message', severity: 3)]
+
+      expect(encoder.encode('message' => messages, 'level_field' => 1))
+        .to include '"level":1'
+      expect(encoder.encode('message' => messages, 'level_field' => 1))
+        .not_to include 'level_field'
+    end
+
+    it 'sets the level to UNKNOWN without any messages' do
+      expect(encoder.encode({})).to include '"level":1'
+    end
+
+    it 'extracts the highest severity from the messages to get the level' do
+      messages = [
+        instance_double('Rackstash::Message', severity: 0), # DEBUG
+        instance_double('Rackstash::Message', severity: 3), # ERROR
+        instance_double('Rackstash::Message', severity: 1), # INFO
+      ]
+
+      expect(encoder.encode('message' => messages)).to include '"level":4'
+    end
+
+    it 'ignores invalid message severities' do
+      messages = [
+        instance_double('Rackstash::Message', severity: 123),
+        instance_double('Rackstash::Message', severity: 5),
+        'a string',
+        32,
+        nil
+      ]
+
+      expect(encoder.encode('message' => messages)).to include '"level":1'
+      expect(encoder.encode('message' => nil)).to include '"level":1'
+      expect(encoder.encode('message' => '')).to include '"level":1'
+    end
+
+    context 'with default_severity' do
+      before do
+        encoder_args[:default_severity] = 1
+      end
+
+      it 'uses the custom default level' do
+        messages = [
+          instance_double('Rackstash::Message', severity: 123),
+          instance_double('Rackstash::Message', severity: 5),
+          instance_double('Rackstash::Message', severity: -3)
+        ]
+
+        expect(encoder.encode('message' => messages)).to include '"level":6'
+        expect(encoder.encode('message' => nil)).to include '"level":6'
+        expect(encoder.encode('message' => '')).to include '"level":6'
+        expect(encoder.encode('message' => [messages.last])).to include '"level":6'
+        expect(encoder.encode({})).to include '"level":6'
+      end
+
+      it 'can set a higher level' do
+        messages = [
+          instance_double('Rackstash::Message', severity: 4)
+        ]
+
+        expect(encoder.encode('message' => messages)).to include '"level":3'
+      end
+    end
+  end
+
+  describe 'short_message field' do
+    it 'adds the event message to the short_message field by default' do
+      expect(encoder.encode('message' => ["Hello\n", "World\n"]))
+        .to include '"short_message":"Hello\nWorld\n"'
+    end
+
+    it 'uses the configured short_message field' do
+      encoder_args[:fields] = { short_message: 'gelf_message' }
+
+      event = {
+        'message' => ["Hello\n", "World\n"],
+        'gelf_message' => 'Hello GELF'
+      }
+
+      expect(encoder.encode(event))
+        .to include('"short_message":"Hello GELF"')
+        .and include('"_message":"Hello\nWorld\n"')
+    end
+
+    it 'sets an empty short_message if the configured field is missing' do
+      encoder_args[:fields] = { short_message: 'gelf_message' }
+
+      expect(encoder.encode({})).to include('"short_message":""')
+    end
+  end
+
+  describe 'full_message field' do
+    it 'does not include the field by default' do
+      expect(encoder.encode({})).not_to include 'full_message'
+    end
+
+    it 'includes the field if configured and present' do
+      encoder_args[:fields] = { full_message: 'full' }
+
+      expect(encoder.encode('full' => 'GELF MESSAGE'))
+        .to include '"full_message":"GELF MESSAGE"'
+    end
+
+    it 'does not include the field if configured and NOT present' do
+      encoder_args[:fields] = { full_message: 'full' }
+
+      expect(encoder.encode({})).not_to include 'full_message'
+    end
+  end
+
+  describe 'additional fields' do
+    it 'adds additional simple fields' do
+      expect(encoder.encode(
+        'str' => 'hello world',
+        'int' => 123,
+        'f' => 3.14,
+        'date' => Date.new(2017, 3, 6),
+        'time' => Time.new(2017, 2, 17, 16, 37, 0, '+03:00'),
+        'datetime' => DateTime.new(2016, 10, 7, 16, 37, 0, '+03:00')
+      ))
+        .to include('"_str":"hello world"')
+        .and include('"_int":123')
+        .and include('"_f":3.14')
+        .and include('"_date":"2017-03-06"')
+        .and include('"_time":"2017-02-17T13:37:00.000000Z"')
+        .and include('"_datetime":"2016-10-07T13:37:00.000000Z"')
+    end
+
+    it 'normalizes keys' do
+      expect(encoder.encode('with spaces' => 'value', 'Motörhead' => "band"))
+        .to include('"_with_spaces":"value"')
+        .and include('"_Mot_rhead":"band"')
+    end
+
+    it "transforms the id key" do
+      expect(encoder.encode('id' => 123, 'nested' => {'_id' => 42}))
+        .to include('"__id":123')
+        .and include('"_nested._id":42')
+    end
+
+    it 'adds nested hashes' do
+      event = {
+        'nested' => {
+          'str' => 'beep',
+          'int' => 123,
+          'inner' => { 'foo' => 'bar' }
+        }
+      }
+
+      expect(encoder.encode(event))
+        .to include('"_nested.str":"beep"')
+        .and include('"_nested.int":123')
+        .and include('"_nested.inner.foo":"bar"')
+    end
+
+    it 'adds nested arrays' do
+      expect(encoder.encode('array' => ['foo', 'bar', [123, 42]]))
+        .to include('"_array.0":"foo"')
+        .and include('"_array.1":"bar"')
+        .and include('"_array.2.0":123')
+        .and include('"_array.2.1":42')
+    end
+  end
+end
diff --git a/spec/rackstash/encoder/helper/fields_map_spec.rb b/spec/rackstash/encoder/helper/fields_map_spec.rb
new file mode 100644
index 0000000..1d3573b
--- /dev/null
+++ b/spec/rackstash/encoder/helper/fields_map_spec.rb
@@ -0,0 +1,64 @@
+# frozen_string_literal: true
+#
+# Copyright 2017 - 2018 Holger Just
+#
+# This software may be modified and distributed under the terms
+# of the MIT license. See the LICENSE.txt file for details.
+
+require 'spec_helper'
+
+require 'rackstash/encoder/helper/fields_map'
+
+RSpec.describe Rackstash::Encoder::Helper::FieldsMap do
+  let(:helper) {
+    helper = Object.new.extend(described_class)
+    described_class.private_instance_methods(false).each do |method|
+      helper.define_singleton_method(method) do |*args, &block|
+        super(*args, &block)
+      end
+    end
+    helper
+  }
+  let(:event) {
+    {
+    'foo' => 'hello',
+    'bar' => 'world'
+    }
+  }
+
+  describe '#extract_field' do
+    context 'with defaults' do
+      let(:defaults) { { default: 'foo' } }
+
+      it 'uses default fields' do
+        helper.set_fields_mapping({ something: 'beep' }, defaults)
+        expect(helper.extract_field(:default, event)).to eql 'hello'
+      end
+
+      it 'can overwrite default fields' do
+        helper.set_fields_mapping({ default: 'bar' }, defaults)
+        expect(helper.extract_field(:default, event)).to eql 'world'
+      end
+    end
+
+    it 'returns the field value if it exists' do
+      helper.set_fields_mapping({}, { field: 'foo' })
+      expect(helper.extract_field(:field, event)).to eql 'hello'
+    end
+
+    it 'returns nil if the field does not exist' do
+      helper.set_fields_mapping({}, { field: 'invalid' })
+      expect(helper.extract_field(:field, event)).to eql nil
+    end
+
+    it 'returns the result of the given block if the field does not exist' do
+      helper.set_fields_mapping({}, { field: 'invalid' })
+      expect(helper.extract_field(:field, event) { 123 }).to eql 123
+    end
+
+    it 'yield the resolved field name' do
+      helper.set_fields_mapping({}, { field: 'field' })
+      expect { |b| helper.extract_field(:field, event, &b) }.to yield_with_args('field')
+    end
+  end
+end