diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 17732cac..042a5829 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -11,6 +11,9 @@ jobs:
 
   build:
     needs: ruby-versions
+    permissions:
+      contents: read
+      checks: write
     name: build (${{ matrix.ruby }} / ${{ matrix.os }})
     strategy:
       matrix:
@@ -29,3 +32,17 @@ jobs:
         rubygems: 3.5.14
     - name: Run test
       run: bundle exec rake test
+
+    - uses: joshmfrankel/simplecov-check-action@main
+      if: matrix.os == 'ubuntu-latest' && github.event_name != 'pull_request'
+      with:
+        check_job_name: "SimpleCov - ${{ matrix.ruby }}"
+        minimum_suite_coverage: 90
+        minimum_file_coverage: 40 # TODO: increase this after switching to SASL::AuthenticationExchange
+        github_token: ${{ secrets.GITHUB_TOKEN }}
+
+    - name: Upload simplecov results
+      uses: actions/upload-artifact@master
+      with:
+        name: coverage-report
+        path: coverage
diff --git a/Gemfile b/Gemfile
index cdf72462..cbceddc9 100644
--- a/Gemfile
+++ b/Gemfile
@@ -13,4 +13,10 @@ gem "rdoc"
 gem "test-unit"
 gem "test-unit-ruby-core", git: "https://github.com/ruby/test-unit-ruby-core"
 
-gem "benchmark-driver"
+gem "benchmark-driver", require: false
+
+group :test do
+  gem "simplecov",        require: false
+  gem "simplecov-html",   require: false
+  gem "simplecov-json",   require: false
+end
diff --git a/test/lib/helper.rb b/test/lib/helper.rb
index bef82346..ce7ab2d6 100644
--- a/test/lib/helper.rb
+++ b/test/lib/helper.rb
@@ -1,3 +1,31 @@
+require "simplecov"
+
+# Cannot use ".simplecov" file: simplecov-json triggers a circular require.
+require "simplecov-json"
+SimpleCov.formatters = SimpleCov::Formatter::MultiFormatter.new([
+  SimpleCov::Formatter::HTMLFormatter,
+  SimpleCov::Formatter::JSONFormatter,
+])
+
+SimpleCov.start do
+  command_name "Net::IMAP tests"
+  enable_coverage  :branch
+  primary_coverage :branch
+  enable_coverage_for_eval
+
+  add_filter "/test/"
+  add_filter "/rakelib/"
+
+  add_group "Parser", %w[lib/net/imap/response_parser.rb
+                         lib/net/imap/response_parser]
+  add_group "Config", %w[lib/net/imap/config.rb
+                         lib/net/imap/config]
+  add_group "SASL", %w[lib/net/imap/sasl.rb
+                       lib/net/imap/sasl
+                       lib/net/imap/authenticators.rb]
+  add_group "StringPrep", %w[lib/net/imap/stringprep.rb
+                             lib/net/imap/stringprep]
+end
 require "test/unit"
 require "core_assertions"