diff --git a/activemodel/CHANGELOG.md b/activemodel/CHANGELOG.md
index 9fcc0f57d6..e0d40a196c 100644
--- a/activemodel/CHANGELOG.md
+++ b/activemodel/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Deprecate marshalling load from legacy attributes format.
+
+    *Ryuta Kamizono*
+
 *   `*_previously_changed?` accepts `:from` and `:to` keyword arguments like `*_changed?`.
 
         topic.update!(status: :archived)
diff --git a/activemodel/lib/active_model/attribute_set/builder.rb b/activemodel/lib/active_model/attribute_set/builder.rb
index 8233a52d28..755662cf50 100644
--- a/activemodel/lib/active_model/attribute_set/builder.rb
+++ b/activemodel/lib/active_model/attribute_set/builder.rb
@@ -78,6 +78,9 @@ module ActiveModel
 
     def marshal_load(values)
       if values.is_a?(Hash)
+        ActiveSupport::Deprecation.warn(<<~MSG.squish)
+          Marshalling load from legacy attributes format is deprecated and will be removed in Rails 6.2.
+        MSG
         empty_hash = {}.freeze
         initialize(empty_hash, empty_hash, empty_hash, empty_hash, values)
         @materialized = true
diff --git a/activemodel/test/cases/attribute_set_test.rb b/activemodel/test/cases/attribute_set_test.rb
index 62feb9074e..6494b9ef40 100644
--- a/activemodel/test/cases/attribute_set_test.rb
+++ b/activemodel/test/cases/attribute_set_test.rb
@@ -224,12 +224,13 @@ module ActiveModel
       attributes.instance_variable_get(:@attributes).instance_eval do
         class << self
           def marshal_dump
-            materialize
+            materialize # legacy marshal format before Rails 5.1
           end
         end
       end
 
-      attributes = Marshal.load(Marshal.dump(attributes))
+      data = Marshal.dump(attributes)
+      attributes = assert_deprecated { Marshal.load(data) }
       assert_equal({ foo: "1" }, attributes.to_hash)
     end