diff --git a/lib/rackstash/filters/replace.rb b/lib/rackstash/filters/replace.rb
new file mode 100644
index 0000000..f574cba
--- /dev/null
+++ b/lib/rackstash/filters/replace.rb
@@ -0,0 +1,52 @@
+# 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.
+
+module Rackstash
+  module Filters
+    # Replace fields in the given event with new values. A new value can be
+    # specified as either a fixed value or as a Proc (or any other object
+    # responding to `call`). In the latter case, the callable object will be
+    # called with the event as its argument. It is then expectd to return the
+    # new value which is set on the key
+    #
+    # @example
+    #   Rackstash::Flow.new(STDOUT) do
+    #     filter :replace, {
+    #       "sample" => ->(event) { "#{event['source_host']}: #{event['sample']}" }
+    #     }
+    #   end
+    #
+    # You should make sure to only set a new object of the basic types here,
+    # namely `String`, `Integer`, `Float`, `nil`, `true`, or `false`.
+    class Replace
+      # @param spec [Hash<#to_s => #call,Object>] a `Hash` specifying new field
+      #   values for the named keys. Values can be given in the form of a fixed
+      #   value or a callable object (e.g. a `Proc`) which accepts the event as
+      #   its argument and returns the new value.
+      def initialize(spec)
+        @replace = {}
+        Hash(spec).each_pair do |key, value|
+          @replace[key.to_s] = value
+        end
+      end
+
+      # Replace field values in the event to a new value. If a specified field
+      # does not exist in the event hah, it will be created with the goven (or
+      # calculated) value anyway.
+      #
+      # @param event [Hash] an event hash
+      # return [Hash] the given `event` with the fields renamed
+      def call(event)
+        @replace.each_pair do |key, value|
+          value = value.call(event) if value.respond_to?(:call)
+          event[key] = value
+        end
+        event
+      end
+    end
+  end
+end
diff --git a/spec/rackstash/filters/replace_spec.rb b/spec/rackstash/filters/replace_spec.rb
new file mode 100644
index 0000000..32db849
--- /dev/null
+++ b/spec/rackstash/filters/replace_spec.rb
@@ -0,0 +1,52 @@
+# 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 'spec_helper'
+
+require 'rackstash/filters/replace'
+
+describe Rackstash::Filters::Replace do
+  let(:event) {
+    {
+      'foo' => 'foo value',
+      'bar' => 'bar value'
+    }
+  }
+
+  def filter!(spec)
+    described_class.new(spec).call(event)
+  end
+
+  it 'sets evaluates values from callable objects' do
+    filter!('foo' => ->(event) { event['foo'].upcase } )
+    expect(event).to eql 'foo' => 'FOO VALUE', 'bar' => 'bar value'
+  end
+
+  it 'sets raw values' do
+    filter!('bar' => 123 )
+    expect(event).to eql 'foo' => 'foo value', 'bar' => 123
+  end
+
+  it 'always sets fields' do
+    filter!('baz' => 42, 'boing' => ->(event) { 'quark' })
+    expect(event).to eql(
+      'foo' => 'foo value',
+      'bar' => 'bar value',
+      'baz' => 42,
+      'boing' => 'quark'
+    )
+  end
+
+  it 'stringifies keys' do
+    filter!(foo: ->(event) { event['foo'].upcase } )
+    expect(event).to eql 'foo' => 'FOO VALUE', 'bar' => 'bar value'
+  end
+
+  it 'returns the given event object' do
+    expect(filter!('bar' => 'baz')).to equal event
+  end
+end