diff --git a/tests/posix/xsi_realtime_threads/CMakeLists.txt b/tests/posix/xsi_realtime_threads/CMakeLists.txt
new file mode 100644
index 0000000000000..8faa9a803471a
--- /dev/null
+++ b/tests/posix/xsi_realtime_threads/CMakeLists.txt
@@ -0,0 +1,11 @@
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(xsi_realtime_threads)
+
+FILE(GLOB app_sources src/*.c)
+
+target_sources(app PRIVATE ${app_sources})
+
+target_compile_options(app PRIVATE -U_POSIX_C_SOURCE -D_POSIX_C_SOURCE=200809L)
diff --git a/tests/posix/xsi_realtime_threads/prj.conf b/tests/posix/xsi_realtime_threads/prj.conf
new file mode 100644
index 0000000000000..b82ef0577c12d
--- /dev/null
+++ b/tests/posix/xsi_realtime_threads/prj.conf
@@ -0,0 +1,18 @@
+CONFIG_ZTEST=y
+
+CONFIG_POSIX_API=y
+
+CONFIG_POSIX_AEP_CHOICE_BASE=y
+CONFIG_XSI_REALTIME_THREADS=y
+CONFIG_ASSERT_VERBOSE=y
+CONFIG_POSIX_THREAD_PRIORITY_SCHEDULING=y
+
+CONFIG_DYNAMIC_THREAD=y
+CONFIG_DYNAMIC_THREAD_POOL_SIZE=6
+CONFIG_DYNAMIC_THREAD_STACK_SIZE=2048
+CONFIG_THREAD_STACK_INFO=y
+CONFIG_HEAP_MEM_POOL_SIZE=4096
+
+CONFIG_LOG=y
+CONFIG_LOG_MODE_IMMEDIATE=y
+CONFIG_LOG_DEFAULT_LEVEL=4
diff --git a/tests/posix/xsi_realtime_threads/src/main.c b/tests/posix/xsi_realtime_threads/src/main.c
new file mode 100644
index 0000000000000..9ba33d788df4d
--- /dev/null
+++ b/tests/posix/xsi_realtime_threads/src/main.c
@@ -0,0 +1,272 @@
+/*
+ * Copyright (c) 2025 Marvin Ouma
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <pthread.h>
+
+#include <zephyr/ztest.h>
+#include <zephyr/sys/util.h>
+
+#define BIOS_FOOD       0xB105F00D
+#define PRIO_INVALID    -1
+#define PTHREAD_INVALID -1
+
+static pthread_attr_t attr;
+static const pthread_attr_t uninit_attr;
+static bool detached_thread_has_finished;
+
+static bool xsi_realtime_threads_predicate(const void *global_state)
+{
+	pthread_attr_init(&attr);
+	pthread_attr_setstacksize(&attr, 1024);
+	return true;
+}
+
+static void xsi_realtime_threads_teardown(void *fixture)
+{
+	pthread_attr_destroy(&attr);
+}
+
+static void *thread_entry(void *arg)
+{
+	bool joinable = (bool)POINTER_TO_UINT(arg);
+
+	if (!joinable) {
+		detached_thread_has_finished = true;
+	}
+
+	return NULL;
+}
+
+static void *inheritsched_entry(void *arg)
+{
+	int prio;
+	int inheritsched;
+	int pprio = POINTER_TO_INT(arg);
+
+	zassert_ok(pthread_attr_getinheritsched(&attr, &inheritsched));
+
+	prio = k_thread_priority_get(k_current_get());
+
+	if (inheritsched == PTHREAD_INHERIT_SCHED) {
+		/*
+		 * There will be numerical overlap between posix priorities in different scheduler
+		 * policies so only check the Zephyr priority here. The posix policy and posix
+		 * priority are derived from the Zephyr priority in any case.
+		 */
+		zassert_equal(prio, pprio, "actual priority: %d, expected priority: %d", prio,
+			      pprio);
+		return NULL;
+	}
+
+	/* inheritsched == PTHREAD_EXPLICIT_SCHED */
+	int act_prio;
+	int exp_prio;
+	int act_policy;
+	int exp_policy;
+	struct sched_param param;
+
+	/* get the actual policy, param, etc */
+	zassert_ok(pthread_getschedparam(pthread_self(), &act_policy, &param));
+	act_prio = param.sched_priority;
+
+	/* get the expected policy, param, etc */
+	zassert_ok(pthread_attr_getschedpolicy(&attr, &exp_policy));
+	zassert_ok(pthread_attr_getschedparam(&attr, &param));
+	exp_prio = param.sched_priority;
+
+	/* compare actual vs expected */
+	zassert_equal(act_policy, exp_policy, "actual policy: %d, expected policy: %d", act_policy,
+		      exp_policy);
+	zassert_equal(act_prio, exp_prio, "actual priority: %d, expected priority: %d", act_prio,
+		      exp_prio);
+
+	return NULL;
+}
+
+static void create_thread_common_entry(const pthread_attr_t *attrp, bool expect_success,
+				       bool joinable, void *(*entry)(void *arg), void *arg)
+{
+	pthread_t th;
+
+	if (!joinable) {
+		detached_thread_has_finished = false;
+	}
+
+	if (expect_success) {
+		zassert_ok(pthread_create(&th, attrp, entry, arg));
+	} else {
+		zassert_not_ok(pthread_create(&th, attrp, entry, arg));
+		return;
+	}
+
+	if (joinable) {
+		zassert_ok(pthread_join(th, NULL), "failed to join joinable thread");
+		return;
+	}
+
+	/* should not be able to join detached thread */
+	zassert_not_ok(pthread_join(th, NULL));
+
+	for (size_t i = 0; i < 10; ++i) {
+		k_msleep(2 * CONFIG_PTHREAD_RECYCLER_DELAY_MS);
+		if (detached_thread_has_finished) {
+			break;
+		}
+	}
+
+	zassert_true(detached_thread_has_finished, "detached thread did not seem to finish");
+}
+
+static void create_thread_common(const pthread_attr_t *attrp, bool expect_success, bool joinable)
+{
+	create_thread_common_entry(attrp, expect_success, joinable, thread_entry,
+				   UINT_TO_POINTER(joinable));
+}
+
+static inline void can_create_thread(const pthread_attr_t *attrp)
+{
+	create_thread_common(attrp, true, true);
+}
+
+static void test_pthread_attr_setinheritsched_common(bool inheritsched)
+{
+	int prio;
+	int policy;
+	struct sched_param param;
+
+	extern int zephyr_to_posix_priority(int priority, int *policy);
+
+	prio = k_thread_priority_get(k_current_get());
+	zassert_not_equal(prio, K_LOWEST_APPLICATION_THREAD_PRIO);
+
+	/*
+	 * values affected by inheritsched are policy / priority / contentionscope
+	 *
+	 * we only support PTHREAD_SCOPE_SYSTEM, so no need to set contentionscope
+	 */
+	prio = K_LOWEST_APPLICATION_THREAD_PRIO;
+	param.sched_priority = zephyr_to_posix_priority(prio, &policy);
+
+	zassert_ok(pthread_attr_setschedpolicy(&attr, policy));
+	zassert_ok(pthread_attr_setschedparam(&attr, &param));
+	zassert_ok(pthread_attr_setinheritsched(&attr, inheritsched));
+	create_thread_common_entry(&attr, true, true, inheritsched_entry,
+				   UINT_TO_POINTER(k_thread_priority_get(k_current_get())));
+}
+
+ZTEST(xsi_realtime_threads, test_pthread_attr_setinheritsched)
+{
+	/* degenerate cases */
+	{
+		if (false) {
+			/* undefined behaviour */
+			zassert_equal(pthread_attr_setinheritsched(NULL, PTHREAD_EXPLICIT_SCHED),
+				      EINVAL);
+			zassert_equal(pthread_attr_setinheritsched(NULL, PTHREAD_INHERIT_SCHED),
+				      EINVAL);
+			zassert_equal(pthread_attr_setinheritsched((pthread_attr_t *)&uninit_attr,
+								   PTHREAD_INHERIT_SCHED),
+				      EINVAL);
+		}
+		zassert_equal(pthread_attr_setinheritsched(&attr, 3), EINVAL);
+	}
+
+	/* valid cases */
+	test_pthread_attr_setinheritsched_common(PTHREAD_INHERIT_SCHED);
+	test_pthread_attr_setinheritsched_common(PTHREAD_EXPLICIT_SCHED);
+}
+
+ZTEST(xsi_realtime_threads, test_pthread_attr_getinheritsched)
+{
+	int inheritsched = BIOS_FOOD;
+
+	/* degenerate cases */
+	{
+		if (false) {
+			/* undefined behaviour */
+			zassert_equal(pthread_attr_getinheritsched(NULL, NULL), EINVAL);
+			zassert_equal(pthread_attr_getinheritsched(NULL, &inheritsched), EINVAL);
+			zassert_equal(pthread_attr_getinheritsched(&uninit_attr, &inheritsched),
+				      EINVAL);
+		}
+		zassert_equal(pthread_attr_getinheritsched(&attr, NULL), EINVAL);
+	}
+
+	zassert_ok(pthread_attr_getinheritsched(&attr, &inheritsched));
+	zassert_equal(inheritsched, PTHREAD_INHERIT_SCHED);
+}
+
+ZTEST(xsi_realtime_threads, test_pthread_attr_getschedparam)
+{
+	struct sched_param param = {
+		.sched_priority = BIOS_FOOD,
+	};
+
+	/* degenerate cases */
+	{
+		if (false) {
+			/* undefined behaviour */
+			zassert_equal(pthread_attr_getschedparam(NULL, NULL), EINVAL);
+			zassert_equal(pthread_attr_getschedparam(NULL, &param), EINVAL);
+			zassert_equal(pthread_attr_getschedparam(&uninit_attr, &param), EINVAL);
+		}
+		zassert_equal(pthread_attr_getschedparam(&attr, NULL), EINVAL);
+	}
+
+	/* only check to see that the function succeeds and sets param */
+	zassert_ok(pthread_attr_getschedparam(&attr, &param));
+	zassert_not_equal(BIOS_FOOD, param.sched_priority);
+}
+
+ZTEST(xsi_realtime_threads, test_pthread_attr_setschedparam)
+{
+	struct sched_param param = {0};
+	/* degenerate cases */
+	{
+		if (false) {
+			/* undefined behaviour */
+			zassert_equal(pthread_attr_setschedparam(NULL, NULL), EINVAL);
+			zassert_equal(pthread_attr_setschedparam(NULL, &param), EINVAL);
+			zassert_equal(
+				pthread_attr_setschedparam((pthread_attr_t *)&uninit_attr, &param),
+				EINVAL);
+		}
+		zassert_equal(pthread_attr_setschedparam(&attr, NULL), EINVAL);
+	}
+
+	zassert_ok(pthread_attr_setschedparam(&attr, &param));
+
+	can_create_thread(&attr);
+}
+
+static void *test_pthread_setschedprio_fn(void *arg)
+{
+	int policy;
+	int prio = 0;
+	struct sched_param param;
+	pthread_t self = pthread_self();
+
+	zassert_equal(pthread_setschedprio(self, PRIO_INVALID), EINVAL, "EINVAL was expected");
+	zassert_equal(pthread_setschedprio(PTHREAD_INVALID, prio), ESRCH, "ESRCH was expected");
+
+	zassert_ok(pthread_setschedprio(self, prio));
+	param.sched_priority = ~prio;
+	zassert_ok(pthread_getschedparam(self, &policy, &param));
+	zassert_equal(param.sched_priority, prio, "Priority unchanged");
+
+	return NULL;
+}
+
+ZTEST(xsi_realtime_threads, test_pthread_setschedprio)
+{
+	pthread_t th;
+
+	zassert_ok(pthread_create(&th, NULL, test_pthread_setschedprio_fn, NULL));
+	zassert_ok(pthread_join(th, NULL));
+}
+
+ZTEST_SUITE(xsi_realtime_threads, xsi_realtime_threads_predicate, NULL, NULL, NULL,
+	    xsi_realtime_threads_teardown);
diff --git a/tests/posix/xsi_realtime_threads/testcase.yaml b/tests/posix/xsi_realtime_threads/testcase.yaml
new file mode 100644
index 0000000000000..4b32ab2ecde78
--- /dev/null
+++ b/tests/posix/xsi_realtime_threads/testcase.yaml
@@ -0,0 +1,31 @@
+common:
+  filter: not CONFIG_NATIVE_LIBC
+  tags:
+    - posix
+    - xsi_realtime_threads
+  # 1 tier0 platform per supported architecture
+  platform_key:
+    - arch
+    - simulation
+  min_flash: 64
+  min_ram: 32
+  timeout: 240
+  platform_exclude:
+    # linker_zephyr_pre0.cmd:140: syntax error (??)
+    - qemu_xtensa/dc233c
+    - native_sim
+    - native_sim/native/64
+tests:
+  portability.posix.xsi_realtime_threads: {}
+  portability.posix.xsi_realtime_threads.minimal:
+    extra_configs:
+      - CONFIG_MINIMAL_LIBC=y
+  portability.posix.xsi_realtime_threads.newlib:
+    filter: TOOLCHAIN_HAS_NEWLIB == 1
+    extra_configs:
+      - CONFIG_NEWLIB_LIBC=y
+  portability.posix.xsi_realtime_threads.picolibc:
+    tags: picolibc
+    filter: CONFIG_PICOLIBC_SUPPORTED
+    extra_configs:
+      - CONFIG_PICOLIBC=y