diff --git a/shared/test/showcase/pqc_test.rb b/shared/test/showcase/pqc_test.rb
new file mode 100644
index 000000000..0e1788ece
--- /dev/null
+++ b/shared/test/showcase/pqc_test.rb
@@ -0,0 +1,270 @@
+# frozen_string_literal: true
+
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+require "test_helper"
+require "google/showcase/v1beta1/echo"
+# Explicit: gapic-common loads GRPC::Core but not grpc/version.rb, so
+# GRPC::VERSION is otherwise undefined.
+require "grpc"
+
+##
+# Verifies that generated Ruby clients negotiate post-quantum hybrid key
+# exchange with the Showcase server.
+#
+# Ruby never performs the key exchange itself: the gRPC transport delegates to
+# the BoringSSL build vendored inside the grpc gem, and the REST transport
+# delegates to the system OpenSSL that Net::HTTP is linked against. These tests
+# therefore assert on what the server observed, using the TLS metadata that
+# Showcase reflects back on every response:
+#
+#   x-showcase-tls-group                    the group that was negotiated
+#   x-showcase-tls-client-supported-groups  everything the client offered
+#
+# Reflecting the server's view keeps the assertions free of any dependence on
+# CRuby's internal OpenSSL object layout, which is not stable across releases
+# or platforms.
+#
+class PqcTest < ShowcaseTest
+  # Header carrying the key exchange group selected during the handshake.
+  NEGOTIATED_GROUP_HEADER = "x-showcase-tls-group"
+
+  # Header carrying every key exchange group the client offered in ClientHello.
+  CLIENT_GROUPS_HEADER = "x-showcase-tls-client-supported-groups"
+
+  # Substring shared by every hybrid post-quantum group name (X25519MLKEM768,
+  # SecP256r1MLKEM768, ...). Matching on it rather than on one exact name keeps
+  # the suite correct as TLS stacks change their preferred hybrid, and mirrors
+  # the .NET conformance tests (googleapis/gax-dotnet#909). No classical group
+  # name contains it, so unlike CLASSICAL_GROUP it cannot produce a false
+  # substring match.
+  PQC_GROUP_MARKER = "MLKEM"
+
+  # The classical group expected once post-quantum groups are withdrawn.
+  CLASSICAL_GROUP = "X25519"
+
+  # IANA codepoints for X25519 and secp256r1. Passed to --tls-groups to strip
+  # every post-quantum group from the server's preferences.
+  CLASSICAL_ONLY_CODEPOINTS = "0x001d,0x0017"
+
+  # grpc 1.83 is the first release whose vendored BoringSSL offers X25519MLKEM768.
+  MINIMUM_GRPC_VERSION = Gem::Version.new "1.83.0"
+
+  # ML-KEM, and therefore X25519MLKEM768, first shipped in OpenSSL 3.5. Ruby's
+  # openssl gem is only a binding, so REST post-quantum support is a property
+  # of the host rather than of any gem we can pin.
+  MINIMUM_REST_OPENSSL_VERSION = Gem::Version.new "3.5.0"
+
+  # Set by CI on jobs whose image is guaranteed to provide OpenSSL >= 3.5. It
+  # does not change which outcome the REST test expects - host capability
+  # decides that - it only asserts that the capability is actually there, so a
+  # base-image regression fails loudly instead of silently turning the REST test
+  # into a classical-only check.
+  REQUIRE_REST_PQC = ENV["SHOWCASE_REQUIRE_REST_PQC"] == "1"
+
+  # Whether the host can perform post-quantum key exchange over REST at all, and
+  # therefore which outcome the REST test expects: a post-quantum group when
+  # true, CLASSICAL_GROUP when false.
+  REST_OPENSSL_SUPPORTS_PQC =
+    Gem::Version.new(OpenSSL::OPENSSL_LIBRARY_VERSION.split[1]) >= MINIMUM_REST_OPENSSL_VERSION
+
+  def test_grpc_negotiates_post_quantum_key_exchange
+    assert_grpc_pqc_capable
+    headers = grpc_tls_headers new_echo_client
+
+    assert_advertises_pqc headers, "gRPC"
+    assert_negotiated_pqc headers, "gRPC"
+  end
+
+  def test_rest_negotiates_post_quantum_key_exchange
+    headers = rest_tls_headers new_echo_rest_client
+
+    assert_rest_key_exchange headers
+  end
+
+  def test_grpc_falls_back_to_classical_key_exchange
+    assert_grpc_pqc_capable
+    with_showcase_tls_groups CLASSICAL_ONLY_CODEPOINTS do |port, ca_path|
+      headers = grpc_tls_headers grpc_echo_client_for(port, ca_path)
+
+      # Negotiating X25519 alone proves nothing: a client that had lost
+      # post-quantum support entirely would produce the same result. What is
+      # being tested is that a PQC-capable client still interoperates with a
+      # classical-only server.
+      assert_advertises_pqc headers, "gRPC"
+      assert_equal CLASSICAL_GROUP, headers[NEGOTIATED_GROUP_HEADER],
+                   "gRPC client failed to fall back to classical key exchange"
+    end
+  end
+
+  def test_rest_falls_back_to_classical_key_exchange
+    with_showcase_tls_groups CLASSICAL_ONLY_CODEPOINTS do |port|
+      headers = rest_tls_headers rest_echo_client_for(port)
+
+      # Same reasoning as the gRPC case, but only checkable where the host
+      # OpenSSL implements ML-KEM at all; below 3.5 the client has no
+      # post-quantum group to withhold, so there is no fallback to observe.
+      assert_advertises_pqc headers, "REST" if REST_OPENSSL_SUPPORTS_PQC
+      assert_equal CLASSICAL_GROUP, headers[NEGOTIATED_GROUP_HEADER],
+                   "REST client failed to fall back to classical key exchange"
+    end
+  end
+
+  private
+
+  ##
+  # Fails if the grpc gem predates the vendored BoringSSL that added
+  # post-quantum key exchange. Nothing here pins grpc - it arrives through
+  # gapic-common - so without this a downgrade looks like a protocol bug rather
+  # than a dependency one.
+  #
+  # @return [void]
+  def assert_grpc_pqc_capable
+    assert_operator Gem::Version.new(GRPC::VERSION), :>=, MINIMUM_GRPC_VERSION,
+                    "grpc #{GRPC::VERSION} predates #{MINIMUM_GRPC_VERSION}, where the " \
+                    "vendored BoringSSL gained X25519MLKEM768"
+  end
+
+  ##
+  # Whether a TLS group name denotes a hybrid post-quantum key exchange.
+  #
+  # @param group [String, nil]
+  # @return [Boolean]
+  def pqc_group? group
+    group.to_s.upcase.include? PQC_GROUP_MARKER
+  end
+
+  ##
+  # Asserts the client offered at least one post-quantum group in its
+  # ClientHello. Checked before the negotiated group so that a failure
+  # distinguishes a client that never offered PQC from a server that declined it.
+  #
+  # @param headers [Hash{String=>String}]
+  # @param transport [String] Label used in failure messages.
+  # @return [void]
+  def assert_advertises_pqc headers, transport
+    offered = offered_groups headers
+    assert offered.any? { |group| pqc_group? group },
+           "#{transport} client did not advertise a post-quantum group in its " \
+           "ClientHello (offered: #{offered.join ', '})"
+  end
+
+  ##
+  # Asserts the server negotiated a post-quantum group.
+  #
+  # @param headers [Hash{String=>String}]
+  # @param transport [String] Label used in failure messages.
+  # @return [void]
+  def assert_negotiated_pqc headers, transport
+    negotiated = headers[NEGOTIATED_GROUP_HEADER]
+    assert pqc_group?(negotiated),
+           "#{transport} negotiated #{negotiated}, not a post-quantum group"
+  end
+
+  ##
+  # Issues an Echo RPC and returns the TLS metadata the server attached to the
+  # response headers, downcased for case-insensitive lookup.
+  #
+  # @param client [Google::Showcase::V1beta1::Echo::Client]
+  # @return [Hash{String=>String}]
+  def grpc_tls_headers client
+    metadata = nil
+    response = client.echo(content: "pqc probe") do |_result, operation|
+      metadata = operation.metadata
+    end
+
+    assert_equal "pqc probe", response.content
+    normalize_headers metadata
+  end
+
+  ##
+  # Issues an Echo REST call and returns the TLS metadata the server attached
+  # to the HTTP response headers, downcased for case-insensitive lookup.
+  #
+  # @param client [Google::Showcase::V1beta1::Echo::Rest::Client]
+  # @return [Hash{String=>String}]
+  def rest_tls_headers client
+    headers = nil
+    response = client.echo(content: "pqc probe") do |_result, operation|
+      headers = operation.underlying_op.headers
+    end
+
+    assert_equal "pqc probe", response.content
+    normalize_headers headers
+  end
+
+  ##
+  # Flattens gRPC metadata and Faraday headers into a single case-insensitive
+  # string map, asserting that the TLS metadata is present at all. Absent
+  # headers mean the request never traversed TLS, which would silently turn
+  # every assertion below into a no-op.
+  #
+  # @param raw [Hash, nil]
+  # @return [Hash{String=>String}]
+  def normalize_headers raw
+    refute_nil raw, "no response metadata was captured"
+    headers = raw.to_h { |key, value| [key.to_s.downcase, Array(value).join(",")] }
+
+    [NEGOTIATED_GROUP_HEADER, CLIENT_GROUPS_HEADER].each do |header|
+      refute_nil headers[header],
+                 "showcase did not report #{header}; the connection was not TLS"
+    end
+    headers
+  end
+
+  ##
+  # The key exchange groups the client advertised, as a list.
+  #
+  # Membership must be tested against the split list, never the raw header
+  # string: CLASSICAL_GROUP ("X25519") is a substring of X25519MLKEM768, so
+  # String#include? would report a match for a group the client never offered.
+  #
+  # @param headers [Hash{String=>String}]
+  # @return [Array<String>]
+  def offered_groups headers
+    headers[CLIENT_GROUPS_HEADER].split(",").map(&:strip)
+  end
+
+  ##
+  # Asserts on the key exchange the server negotiated for a REST call.
+  #
+  # The gRPC transport carries its own BoringSSL inside the grpc gem, so it can
+  # be held to post-quantum key exchange unconditionally. REST cannot: it
+  # delegates to the host's OpenSSL, and ML-KEM only exists from OpenSSL 3.5
+  # onward. Rather than skipping on older hosts, which would leave REST
+  # unverified on the stock GitHub Actions runner, the expected outcome follows
+  # from the host: a post-quantum group on OpenSSL >= 3.5, CLASSICAL_GROUP below
+  # it. Either way exactly one outcome passes.
+  #
+  # @param headers [Hash{String=>String}]
+  # @return [void]
+  def assert_rest_key_exchange headers
+    if REQUIRE_REST_PQC
+      assert REST_OPENSSL_SUPPORTS_PQC,
+             "SHOWCASE_REQUIRE_REST_PQC is set but the host provides " \
+             "#{OpenSSL::OPENSSL_LIBRARY_VERSION}; post-quantum key exchange " \
+             "requires OpenSSL >= #{MINIMUM_REST_OPENSSL_VERSION}"
+    end
+
+    if REST_OPENSSL_SUPPORTS_PQC
+      assert_advertises_pqc headers, "REST"
+      assert_negotiated_pqc headers, "REST"
+    else
+      assert_equal CLASSICAL_GROUP, headers[NEGOTIATED_GROUP_HEADER],
+                   "REST on #{OpenSSL::OPENSSL_LIBRARY_VERSION} should negotiate " \
+                   "#{CLASSICAL_GROUP}"
+    end
+  end
+end
diff --git a/shared/test/showcase/test_helper.rb b/shared/test/showcase/test_helper.rb
index b08fefcb5..55d4ddcec 100644
--- a/shared/test/showcase/test_helper.rb
+++ b/shared/test/showcase/test_helper.rb
@@ -173,6 +173,79 @@ def new_compliance_rest_client
     end
   end
 
+  # Echo client for an auxiliary server started by with_showcase_tls_groups.
+  #
+  # @param port [Integer]
+  # @param ca_path [String] CA certificate minted by that server.
+  # @return [Google::Showcase::V1beta1::Echo::Client]
+  def grpc_echo_client_for port, ca_path
+    Google::Showcase::V1beta1::Echo::Client.new do |config|
+      config.endpoint = "localhost:#{port}"
+      config.credentials = ShowcaseTest.channel_credentials ca_path
+    end
+  end
+
+  # Echo REST client for an auxiliary server started by
+  # with_showcase_tls_groups. Trust comes from SSL_CERT_FILE, which that helper
+  # points at the server's CA for the duration of its block.
+  #
+  # @param port [Integer]
+  # @return [Google::Showcase::V1beta1::Echo::Rest::Client]
+  def rest_echo_client_for port
+    Google::Showcase::V1beta1::Echo::Rest::Client.new do |config|
+      config.endpoint = "https://localhost:#{port}"
+      config.credentials = :this_channel_is_insecure
+    end
+  end
+
+  ##
+  # Boots an auxiliary Showcase server whose key exchange preferences are
+  # restricted to the given IANA codepoints, yields its port and the CA
+  # certificate it generated, and guarantees the process is reaped.
+  #
+  # Every server started with --tls mints its own certificate authority, so the
+  # auxiliary server cannot share a trust root with the main harness. Both
+  # transports have to be pointed at the CA yielded here: REST through
+  # SSL_CERT_FILE, which is scoped to the block below because Net::HTTP rebuilds
+  # its trust store per connection, and gRPC through explicit credentials built
+  # from the yielded path.
+  #
+  # @param codepoints [String] Comma separated IANA key exchange group IDs.
+  # @yieldparam port [Integer]
+  # @yieldparam ca_path [String]
+  # @return [void]
+  def with_showcase_tls_groups codepoints
+    # This helper is only safe when tests run one at a time. It repoints the
+    # process-wide SSL_CERT_FILE, which every concurrent REST connection would
+    # pick up, and restores it on exit, which would clobber an overlapping
+    # caller's value. It also binds a fixed port and CA file path, so two
+    # overlapping calls would collide. Minitest runs serially by default;
+    # parallelize_me! switches the class's test_order to :parallel.
+    refute_equal :parallel, self.class.test_order,
+                 "#{self.class} is parallelized, but with_showcase_tls_groups " \
+                 "mutates process-wide state and must run serially"
+
+    dir = ShowcaseTest.instance_variable_get :@showcase_dir
+    skip "requires a showcase server managed by this test run" if dir.nil?
+
+    port = SHOWCASE_PORT + 1
+    ca_path = File.join dir, "ca-#{port}.pem"
+    pid = spawn_showcase "#{dir}/gapic-showcase",
+                         port: port,
+                         ca_path: ca_path,
+                         log_file: File.join(dir, "gapic-showcase-#{port}.log"),
+                         extra_args: ["--tls-groups", codepoints]
+
+    original_tls_env = TLS_ENV_KEYS.to_h { |key| [key, ENV[key]] }
+    begin
+      TLS_ENV_KEYS.each { |key| ENV[key] = ca_path }
+      yield port, ca_path
+    ensure
+      original_tls_env.each { |key, value| ENV[key] = value }
+      stop_showcase pid
+    end
+  end
+
   # Env vars pointing REST at Showcase's CA, saved so after_run can restore them
   # rather than leak a test-only root. REST only: Net::HTTP re-reads
   # SSL_CERT_FILE per connection, while the gRPC C core resolves