diff --git a/lib/rubygems/specification.rb b/lib/rubygems/specification.rb
index 3c8248793c..978854d75f 100644
--- a/lib/rubygems/specification.rb
+++ b/lib/rubygems/specification.rb
@@ -732,6 +732,7 @@ class Gem::Specification < Gem::BasicSpecification
   # Formerly used to set rubyforge project.
 
   attr_writer :rubyforge_project
+  deprecate :rubyforge_project=, :none,       2019, 12
 
   ##
   # The Gem::Specification version of this gemspec.