diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 9c7078b9a494f66622708207e65e296a95cb29c4..172c6972a6150a49d69335ea8e2fd28b523e235d 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -19,6 +19,22 @@
 
     *Godfrey Chan*
 
+*   Removed the old pure-Ruby JSON encoder and switched to a new encoder based on the built-in JSON
+    gem.
+
+    Support for encoding `BigDecimal` as a JSON number, as well as defining custom `encode_json`
+    methods to control the JSON output has been **removed from core**. The new encoder will always
+    encode BigDecimals as `String`s and ignore any custom `encode_json` methods.
+
+    The old encoder has been extracted into the `activesupport-json_encoder` gem. Installing that
+    gem will bring back the ability to encode `BigDecimal`s as numbers as well as `encode_json`
+    support.
+
+    Setting the related configuration `ActiveSupport.encode_big_decimal_as_string` without the
+    `activesupport-json_encoder` gem installed will raise an error.
+
+    *Godfrey Chan*
+
 *   Add `ActiveSupport::Testing::TimeHelpers#travel` and `#travel_to`. These methods change current
     time to the given time or time difference by stubbing `Time.now` and `Date.today` to return the
     time or date after the difference calculation, or the time or date that got passed into the
diff --git a/activesupport/lib/active_support/json/encoding.rb b/activesupport/lib/active_support/json/encoding.rb
index eb25ef7a4c7cee2ac39af697a6916574eb7bdb32..060dcb6995ed84db4d9c0659231bacc22adabe2f 100644
--- a/activesupport/lib/active_support/json/encoding.rb
+++ b/activesupport/lib/active_support/json/encoding.rb
@@ -5,6 +5,7 @@ module ActiveSupport
   class << self
     delegate :use_standard_json_time_format, :use_standard_json_time_format=,
       :escape_html_entities_in_json, :escape_html_entities_in_json=,
+      :encode_big_decimal_as_string, :encode_big_decimal_as_string=,
       :json_encoder, :json_encoder=,
       :to => :'ActiveSupport::JSON::Encoding'
   end
@@ -113,6 +114,32 @@ class << self
         # in +Object#to_json+ and +ActiveSupport::JSON.encode+.
         attr_accessor :json_encoder
 
+        def encode_big_decimal_as_string=(as_string)
+          message = \
+            "The JSON encoder in Rails 4.1 no longer supports encoding BigDecimals as JSON numbers. Instead, " \
+            "the new encoder will always encode them as strings.\n\n" \
+            "You are seeing this error because you have 'active_support.encode_big_decimal_as_string' in " \
+            "your configuration file. If you have been setting this to true, you can safely remove it from " \
+            "your configuration. Otherwise, you should add the 'activesupport-json_encoder' gem to your " \
+            "Gemfile in order to restore this functionality."
+
+          raise NotImplementedError, message
+        end
+
+        def encode_big_decimal_as_string
+          message = \
+            "The JSON encoder in Rails 4.1 no longer supports encoding BigDecimals as JSON numbers. Instead, " \
+            "the new encoder will always encode them as strings.\n\n" \
+            "You are seeing this error because you are trying to check the value of the related configuration, " \
+            "'active_support.encode_big_decimal_as_string'. If your application depends on this option, you should " \
+            "add the 'activesupport-json_encoder' gem to your Gemfile. For now, this option will always be true. " \
+            "In the future, it will be removed from Rails, so you should stop checking its value."
+
+          ActiveSupport::Deprecation.warn message
+
+          true
+        end
+
         # Deprecate CircularReferenceError
         def const_missing(name)
           if name == :CircularReferenceError
diff --git a/activesupport/test/json/encoding_test.rb b/activesupport/test/json/encoding_test.rb
index 79e639b5088a690b1533920aa02dbea907f25269..78cf4819f93565468ac42b688589616a62b67483 100644
--- a/activesupport/test/json/encoding_test.rb
+++ b/activesupport/test/json/encoding_test.rb
@@ -172,6 +172,22 @@ def test_wide_utf8_roundtrip
     assert_equal "𐒑", decoded_hash['string']
   end
 
+  def test_reading_encode_big_decimal_as_string_option
+    assert_deprecated do
+      assert ActiveSupport.encode_big_decimal_as_string
+    end
+  end
+
+  def test_setting_deprecated_encode_big_decimal_as_string_option
+    assert_raise(NotImplementedError) do
+      ActiveSupport.encode_big_decimal_as_string = true
+    end
+
+    assert_raise(NotImplementedError) do
+      ActiveSupport.encode_big_decimal_as_string = false
+    end
+  end
+
   def test_exception_raised_when_encoding_circular_reference_in_array
     a = [1]
     a << a