diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 51b085b..a102a11 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -7,7 +7,7 @@ jobs:
     name: build (${{ matrix.ruby }} / ${{ matrix.os }})
     strategy:
       matrix:
-        ruby: [ 2.7, 2.6, 2.5, 2.4, head ]
+        ruby: [ 2.7, 2.6, head ]
         os: [ ubuntu-latest, macos-latest ]
     runs-on: ${{ matrix.os }}
     steps:
diff --git a/open3.gemspec b/open3.gemspec
index 65ddaa4..0c23648 100644
--- a/open3.gemspec
+++ b/open3.gemspec
@@ -14,6 +14,7 @@ Gem::Specification.new do |spec|
   spec.description   = spec.summary
   spec.homepage      = "https://github.com/ruby/open3"
   spec.license       = "BSD-2-Clause"
+  spec.required_ruby_version = ">= 2.6.0"
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage