diff --git a/.github/workflows/lint.yml b/.github/workflows/lint.yml
index c093202..3d34fc5 100644
--- a/.github/workflows/lint.yml
+++ b/.github/workflows/lint.yml
@@ -15,7 +15,7 @@ jobs:
     - name: Set up Ruby
       uses: ruby/setup-ruby@9eb537ca036ebaed86729dcb9309076e4c5c3b74 # ruby/setup-ruby@v1 see https://github.com/ruby/setup-ruby/tree/v1
       with:
-        ruby-version: 3.4
+        ruby-version: 4.0
     - name: Install dependencies
       run: bundle install
     - name: Run RuboCop against BASE..HEAD changes
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 6aff3a0..1eadcab 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -8,13 +8,14 @@ jobs:
       fail-fast: false
       matrix:
         ruby-version:
-          - '3.2'
           - '3.3'
           - '3.4'
+          - '4.0'
         gemfile:
           - gemfiles/Gemfile.rails71
           - gemfiles/Gemfile.rails72
           - gemfiles/Gemfile.rails80
+          - gemfiles/Gemfile.rails81
         exclude:
           # rails 8.0 requires ruby >= 3.2
           # https://www.fastruby.io/blog/ruby/rails/versions/compatibility-table.html
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 86e4f72..46af3ec 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,6 @@
 ## [Unreleased]
+### Fixed
+* Support Ruby 4.0, Rails 8.1. Drop support for Ruby 3.2
 
 ## 0.3.1 / 2026-03-05
 ### Fixed
diff --git a/gemfiles/Gemfile.rails81 b/gemfiles/Gemfile.rails81
new file mode 100644
index 0000000..195915c
--- /dev/null
+++ b/gemfiles/Gemfile.rails81
@@ -0,0 +1,5 @@
+source 'https://rubygems.org'
+
+gemspec path: '..'
+
+gem 'activesupport', '~> 8.1.0'
diff --git a/ndr_lookup.gemspec b/ndr_lookup.gemspec
index b7e8dc7..ded7443 100644
--- a/ndr_lookup.gemspec
+++ b/ndr_lookup.gemspec
@@ -2,6 +2,9 @@ lib = File.expand_path('lib', __dir__)
 $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
 require 'ndr_lookup/version'
 
+# We list development dependencies for all Rails versions here.
+# Rails version-specific dependencies can go in the relevant Gemfile.
+# rubocop:disable Gemspec/DevelopmentDependencies
 Gem::Specification.new do |spec|
   spec.name          = 'ndr_lookup'
   spec.version       = NdrLookup::VERSION
@@ -22,15 +25,16 @@ Gem::Specification.new do |spec|
   spec.require_paths = ['lib']
 
   spec.add_dependency 'activeresource', '>= 6.0', '< 7'
-  spec.add_dependency 'activesupport', '>= 7.1', '< 8.1'
+  spec.add_dependency 'activesupport', '>= 7.1', '< 8.2'
   spec.add_dependency 'httpi', '~> 4.0'
   spec.add_dependency 'rubyntlm', '~> 0.6'
 
-  spec.add_development_dependency 'bundler', '~> 2.0'
-  spec.add_development_dependency 'minitest', '~> 5.0'
+  spec.add_development_dependency 'bundler', '>= 2.0', '< 5.0'
+  spec.add_development_dependency 'minitest', '>= 5.0.0'
   spec.add_development_dependency 'ndr_dev_support', '>= 6.0', '< 8.0'
   spec.add_development_dependency 'rake', '>= 12.3.3'
   spec.add_development_dependency 'webmock'
 
-  spec.required_ruby_version = '>= 3.2'
+  spec.required_ruby_version = '>= 3.3'
 end
+# rubocop:enable Gemspec/DevelopmentDependencies