diff --git a/test_security/CMakeLists.txt b/test_security/CMakeLists.txt
index b0a08b7e..2b076f10 100644
--- a/test_security/CMakeLists.txt
+++ b/test_security/CMakeLists.txt
@@ -91,6 +91,17 @@ if(BUILD_TESTING)
     set(TEST_PUBLISHER_EXECUTABLE "$<TARGET_FILE:test_secure_publisher_cpp>")
     set(TEST_SUBSCRIBER_EXECUTABLE "$<TARGET_FILE:test_secure_subscriber_cpp>")
 
+    # On Windows, FastDDS SHM transport causes an Access Violation (exit code 0xC0000005)
+    # when sending large secured messages (e.g. UnboundedSequences). Inject a FastDDS XML
+    # profile that forces UDPv4 transport to prevent the crash. Empty on non-Windows
+    # platforms so no behaviour change occurs there. (Issue #561)
+    set(FASTDDS_PROFILES_FILE "")
+    if(WIN32)
+      file(TO_NATIVE_PATH
+        "${CMAKE_CURRENT_SOURCE_DIR}/test/test_security_files/fastdds_no_shm.xml"
+        FASTDDS_PROFILES_FILE)
+    endif()
+
     # Test suite for communication without security
     set(non_secure_comm_PUBLISHER_ROS_SECURITY_ENABLE_LIST "false;false;false;true;false")
     set(non_secure_comm_SUBSCRIBER_ROS_SECURITY_ENABLE_LIST "false;false;false;false;true")
diff --git a/test_security/test/test_secure_publisher_subscriber.py.in b/test_security/test/test_secure_publisher_subscriber.py.in
index 53cef7f0..025f5cba 100644
--- a/test_security/test/test_secure_publisher_subscriber.py.in
+++ b/test_security/test/test_secure_publisher_subscriber.py.in
@@ -47,6 +47,13 @@ def generate_test_description():
     publisher_env['ROS_SECURITY_KEYSTORE'] = '@PUBLISHER_ROS_SECURITY_KEYSTORE@'
     if '@PUBLISHER_RMW@' == 'rmw_connextdds':
         publisher_env['RTI_MONITORING2_ENABLE'] = 'false'
+    # On Windows, FastDDS SHM transport causes an Access Violation (0xC0000005) when
+    # sending large messages (e.g. UnboundedSequences) with security enabled.
+    # Force UDPv4 transport via a FastDDS XML profile to prevent the crash. (Issue #561)
+    if '@PUBLISHER_RMW@' in ('rmw_fastrtps_cpp', 'rmw_fastrtps_dynamic_cpp'):
+        if '@FASTDDS_PROFILES_FILE@':
+            publisher_env['FASTRTPS_DEFAULT_PROFILES_FILE'] = '@FASTDDS_PROFILES_FILE@'
+            publisher_env['RMW_FASTRTPS_USE_QOS_FROM_XML'] = '1'
 
     launch_description.add_action(ExecuteProcess(
         cmd=publisher_cmd,
@@ -61,6 +68,11 @@ def generate_test_description():
     subscriber_env['ROS_SECURITY_KEYSTORE'] = '@SUBSCRIBER_ROS_SECURITY_KEYSTORE@'
     if '@SUBSCRIBER_RMW@' == 'rmw_connextdds':
         subscriber_env['RTI_MONITORING2_ENABLE'] = 'false'
+    # See publisher comment above. (Issue #561)
+    if '@SUBSCRIBER_RMW@' in ('rmw_fastrtps_cpp', 'rmw_fastrtps_dynamic_cpp'):
+        if '@FASTDDS_PROFILES_FILE@':
+            subscriber_env['FASTRTPS_DEFAULT_PROFILES_FILE'] = '@FASTDDS_PROFILES_FILE@'
+            subscriber_env['RMW_FASTRTPS_USE_QOS_FROM_XML'] = '1'
 
     subscriber_process = ExecuteProcess(
         cmd=subscriber_cmd,
diff --git a/test_security/test/test_security_files/fastdds_no_shm.xml b/test_security/test/test_security_files/fastdds_no_shm.xml
new file mode 100644
index 00000000..15ff9ff2
--- /dev/null
+++ b/test_security/test/test_security_files/fastdds_no_shm.xml
@@ -0,0 +1,33 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  FastDDS transport profile used by test_secure_publisher_subscriber tests on Windows.
+
+  On Windows, FastDDS uses Shared Memory (SHM) transport by default for intra-host
+  communication. When DDS-Security is enabled, the security overhead can cause the SHM
+  segment to overflow when sending large messages (e.g. UnboundedSequences), resulting
+  in an Access Violation crash (exit code 0xC0000005) in the publisher process.
+
+  This profile disables SHM and falls back to UDPv4 with enlarged send/receive buffers
+  to accommodate the security-layer overhead on large messages.
+
+  See: https://github.com/ros2/system_tests/issues/561
+-->
+<profiles xmlns="http://www.eprosima.com/XMLSchemas/fastRTPS_Profiles">
+  <transport_descriptors>
+    <transport_descriptor>
+      <transport_id>udpv4_security</transport_id>
+      <type>UDPv4</type>
+      <sendBufferSize>1048576</sendBufferSize>
+      <receiveBufferSize>1048576</receiveBufferSize>
+    </transport_descriptor>
+  </transport_descriptors>
+
+  <participant profile_name="default_participant_profile" is_default_profile="true">
+    <rtps>
+      <userTransports>
+        <transport_id>udpv4_security</transport_id>
+      </userTransports>
+      <useBuiltinTransports>false</useBuiltinTransports>
+    </rtps>
+  </participant>
+</profiles>