diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ec5655d57..7d042c383 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -45,3 +45,63 @@ jobs:
       - name: Run CI
         run: |
           toys ci
+
+  # The showcase suite above runs on ubuntu-latest, whose system OpenSSL is
+  # 3.0.13. ML-KEM only exists from OpenSSL 3.5, so the REST test there
+  # expects the classical X25519 fallback. gRPC is unaffected because it
+  # carries its own BoringSSL inside the grpc gem.
+  #
+  # This job re-runs the same suite on a Debian trixie image (OpenSSL 3.5.5),
+  # where the REST test expects a post-quantum group instead. It is what
+  # actually proves REST post-quantum key exchange works. SHOWCASE_REQUIRE_REST_PQC
+  # makes the suite fail if the image's OpenSSL ever drops below 3.5, rather
+  # than silently falling back to the classical expectation.
+  #
+  # Once ubuntu-latest moves to Ubuntu 26.04 (OpenSSL 3.5.5) this job can be
+  # dropped and SHOWCASE_REQUIRE_REST_PQC set on the matrix above instead. That
+  # migration starts 2026-10-19 (actions/runner-images#14748). Tracked in
+  # b/564510736 - check the runner's actual OpenSSL before removing this,
+  # rather than going by the announced dates.
+  pqc-rest:
+    if: ${{ github.repository == 'googleapis/gapic-generator-ruby' }}
+    runs-on: ubuntu-latest
+    container: ruby:3.2-trixie
+    env:
+      SHOWCASE_REQUIRE_REST_PQC: "1"
+    steps:
+      # Deliberately no ruby/setup-ruby: inside a container it detects
+      # debian-13-x64, treats the job as self-hosted, and will not use the
+      # prebuilt binaries. The image's own Ruby is already linked against the
+      # OpenSSL we want.
+      - name: Checkout repo
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
+        with:
+          persist-credentials: false
+      - name: Checkout submodules
+        run: |
+          # actions/checkout writes as the runner user (uid 1001) but this
+          # container runs as root, so git rejects the tree as dubiously
+          # owned. The host-runner `tests` job above does not hit this.
+          git config --global --add safe.directory '*'
+          git submodule set-url shared/googleapis https://github.com/googleapis/googleapis.git
+          git submodule set-url shared/gapic-showcase https://github.com/googleapis/gapic-showcase.git
+          git submodule update --init --recursive
+      - name: Verify the image provides a post-quantum capable OpenSSL
+        run: |
+          ruby -ropenssl -e '
+            required = Gem::Version.new "3.5.0"
+            actual = Gem::Version.new OpenSSL::OPENSSL_LIBRARY_VERSION.split[1]
+            if actual < required
+              abort "This job asserts post-quantum key exchange but the image " \
+                    "provides OpenSSL #{actual}, below the required #{required}."
+            end
+            puts "OpenSSL #{actual} supports post-quantum key exchange."
+          '
+      - name: Install dependencies
+        run: |
+          gem install --no-document toys
+          toys bundle install
+      - name: Run showcase tests
+        run: |
+          cd shared
+          toys test showcase