diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 6272dd3471..cd96f4b750 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Add `ActiveSupport::HashWithIndifferentAccess#assoc`.
+
+    `assoc` can now be called with either a string or a symbol.
+
+    *Stefan Schüßler*
+
 ## Rails 6.0.0.beta1 (January 18, 2019) ##
 
 *   Remove deprecated `Module#reachable?` method.
diff --git a/activesupport/lib/active_support/hash_with_indifferent_access.rb b/activesupport/lib/active_support/hash_with_indifferent_access.rb
index f1af76019a..3a2b2652c4 100644
--- a/activesupport/lib/active_support/hash_with_indifferent_access.rb
+++ b/activesupport/lib/active_support/hash_with_indifferent_access.rb
@@ -164,6 +164,19 @@ module ActiveSupport
       super(convert_key(key))
     end
 
+    # Same as <tt>Hash#assoc</tt> where the key passed as argument can be
+    # either a string or a symbol:
+    #
+    #   counters = ActiveSupport::HashWithIndifferentAccess.new
+    #   counters[:foo] = 1
+    #
+    #   counters.assoc('foo') # => ["foo", 1]
+    #   counters.assoc(:foo)  # => ["foo", 1]
+    #   counters.assoc(:zoo)  # => nil
+    def assoc(key)
+      super(convert_key(key))
+    end
+
     # Same as <tt>Hash#fetch</tt> where the key passed as argument can be
     # either a string or a symbol:
     #
diff --git a/activesupport/test/hash_with_indifferent_access_test.rb b/activesupport/test/hash_with_indifferent_access_test.rb
index f81e0dc70f..8a39672609 100644
--- a/activesupport/test/hash_with_indifferent_access_test.rb
+++ b/activesupport/test/hash_with_indifferent_access_test.rb
@@ -447,6 +447,14 @@ class HashWithIndifferentAccessTest < ActiveSupport::TestCase
     assert_instance_of ActiveSupport::HashWithIndifferentAccess, indifferent_strings
   end
 
+  def test_indifferent_assoc
+    indifferent_strings = ActiveSupport::HashWithIndifferentAccess.new(@strings)
+    key, value = indifferent_strings.assoc(:a)
+
+    assert_equal("a", key)
+    assert_equal(1, value)
+  end
+
   def test_indifferent_compact
     hash_contain_nil_value = @strings.merge("z" => nil)
     hash = ActiveSupport::HashWithIndifferentAccess.new(hash_contain_nil_value)