diff --git a/LICENSE.rdoc b/LICENSE.rdoc
index c61bb93398..5e498115ec 100644
--- a/LICENSE.rdoc
+++ b/LICENSE.rdoc
@@ -1,3 +1,7 @@
+:stopdoc:
+SPDX-License-Identifier: Ruby or GPL-2.0-only
+:startdoc:
+
 = License
 
 RDoc is copyrighted free software.
diff --git a/rdoc.gemspec b/rdoc.gemspec
index 26340c030e..1afe52f7b6 100644
--- a/rdoc.gemspec
+++ b/rdoc.gemspec
@@ -26,7 +26,7 @@ RDoc produces HTML and command-line documentation for Ruby projects.
 RDoc includes the +rdoc+ and +ri+ tools for generating and displaying documentation from the command-line.
   DESCRIPTION
   s.homepage = "https://ruby.github.io/rdoc"
-  s.licenses = ["Ruby"]
+  s.licenses = ["Ruby", "GPL-2.0-only"]
 
   s.metadata["homepage_uri"] = s.homepage
   s.metadata["source_code_uri"] = "https://github.com/ruby/rdoc"