diff --git a/activesupport/lib/active_support/core_ext/object/deep_dup.rb b/activesupport/lib/active_support/core_ext/object/deep_dup.rb
index c66c5eb2d9..5c903917f5 100644
--- a/activesupport/lib/active_support/core_ext/object/deep_dup.rb
+++ b/activesupport/lib/active_support/core_ext/object/deep_dup.rb
@@ -43,7 +43,7 @@ class Hash
   def deep_dup
     hash = dup
     each_pair do |key, value|
-      if key.frozen? && ::String === key
+      if (::String === key && key.frozen?) || ::Symbol === key
         hash[key] = value.deep_dup
       else
         hash.delete(key)
diff --git a/activesupport/test/core_ext/object/deep_dup_test.rb b/activesupport/test/core_ext/object/deep_dup_test.rb
index 1fef3129c6..a4fa00f5f2 100644
--- a/activesupport/test/core_ext/object/deep_dup_test.rb
+++ b/activesupport/test/core_ext/object/deep_dup_test.rb
@@ -56,4 +56,14 @@ class DeepDupTest < ActiveSupport::TestCase
     dup = hash.deep_dup
     assert_equal 1, dup.keys.length
   end
+
+  def test_deep_dup_with_mutable_frozen_key
+    key = { array: [] }.freeze
+    hash = { key => :value }
+
+    dup = hash.deep_dup
+    dup.transform_keys { |k| k[:array] << :array_element }
+
+    assert_not_equal hash.keys, dup.keys
+  end
 end