diff --git a/core/libs/prx/libSceUlt/CMakeLists.txt b/core/libs/prx/libSceUlt/CMakeLists.txt
index 56c1216ca..e9ce2a507 100644
--- a/core/libs/prx/libSceUlt/CMakeLists.txt
+++ b/core/libs/prx/libSceUlt/CMakeLists.txt
@@ -15,4 +15,17 @@ if(BUILD_TESTING)
                 "PATH=path_list_prepend:$<TARGET_FILE_DIR:libSceUlt>")
         endif()
     endforeach()
+
+    add_test_executable(guest_ult_thread_tests ${LIBS_INCLUDE_DIR}/tests/GuestUltThread.cpp)
+    target_include_directories(guest_ult_thread_tests PRIVATE ${LIBS_INCLUDE_DIR})
+    target_link_libraries(guest_ult_thread_tests PRIVATE libSceUlt libc)
+    configure_windows_unwind(guest_ult_thread_tests)
+    foreach(testCase IN ITEMS outstanding validation)
+        add_test(NAME guest_ult_thread_${testCase} COMMAND guest_ult_thread_tests ${testCase})
+        set_tests_properties(guest_ult_thread_${testCase} PROPERTIES TIMEOUT 20)
+        if(WIN32)
+            set_tests_properties(guest_ult_thread_${testCase} PROPERTIES ENVIRONMENT_MODIFICATION
+                "PATH=path_list_prepend:$<TARGET_FILE_DIR:libSceUlt>")
+        endif()
+    endforeach()
 endif()
diff --git a/core/libs/prx/libSceUlt/Export.cpp b/core/libs/prx/libSceUlt/Export.cpp
index a224d3c35..979033e5a 100644
--- a/core/libs/prx/libSceUlt/Export.cpp
+++ b/core/libs/prx/libSceUlt/Export.cpp
@@ -123,6 +123,9 @@ int APS5_VABI sceUltFinalize() {
     std::vector<std::shared_ptr<UltSemaphoreState>> semaphores;
     {
         std::lock_guard<std::mutex> lock(gMutex);
+        if (!gUlthreads.empty()) {
+            return ULT_ERROR_BUSY;
+        }
         semaphores.reserve(gSemaphores.size());
         for (auto& entry : gSemaphores) {
             auto& state = entry.second;
@@ -143,7 +146,6 @@ int APS5_VABI sceUltFinalize() {
         gQueues.clear();
         gQueueDataPools.clear();
         gRuntimes.clear();
-        gUlthreads.clear();
     }
     for (const auto& state : semaphores) {
         state->_available.notify_all();
diff --git a/core/libs/tests/GuestUltThread.cpp b/core/libs/tests/GuestUltThread.cpp
new file mode 100644
index 000000000..e46308a40
--- /dev/null
+++ b/core/libs/tests/GuestUltThread.cpp
@@ -0,0 +1,94 @@
+#include "SceTypes.hpp"
+#include <atomic>
+#include <cstdio>
+#include <cstdlib>
+#include <cstdint>
+#include <cstring>
+#include <thread>
+
+extern "C" {
+int APS5_VABI sceUltInitialize();
+int APS5_VABI sceUltFinalize();
+std::uint64_t APS5_VABI sceUltUlthreadRuntimeGetWorkAreaSize(std::uint32_t, std::uint32_t);
+int APS5_VABI sceUltUlthreadRuntimeCreate(void*, const char*, std::uint32_t, std::uint32_t, void*, const void*, std::uint32_t);
+int APS5_VABI sceUltUlthreadCreate(void*, const char*, UltUlthreadEntry, std::uint64_t, void*, std::uint64_t, void*, const void*, std::uint32_t);
+int APS5_VABI sceUltUlthreadJoin(void*, std::int32_t*);
+}
+
+static constexpr int Ok = 0;
+static constexpr int Null = -2139029503;
+static constexpr int State = -2139029498;
+static constexpr int Busy = -2139029497;
+
+static void Require(bool value, int line) {
+    if (!value) {
+        std::fprintf(stderr, "ULT thread check failed at line %d\n", line);
+        std::abort();
+    }
+}
+
+#define CHECK(value) Require((value), __LINE__)
+
+static std::atomic<bool> gStarted{false};
+static std::atomic<bool> gRelease{false};
+
+static std::int32_t APS5_VABI Parked(std::uint64_t arg) {
+    gStarted.store(true, std::memory_order_release);
+    while (!gRelease.load(std::memory_order_acquire)) std::this_thread::yield();
+    return static_cast<std::int32_t>(arg);
+}
+
+static std::int32_t APS5_VABI Immediate(std::uint64_t arg) {
+    return static_cast<std::int32_t>(arg);
+}
+
+struct alignas(8) Runtime { std::uint8_t bytes[4096]{}; };
+struct alignas(8) Thread { std::uint8_t bytes[512]{}; };
+
+static void Outstanding() {
+    Runtime runtime;
+    Thread thread;
+    CHECK(sceUltUlthreadRuntimeCreate(&runtime, "runtime", 1, 1, nullptr, nullptr, 0) == Ok);
+    CHECK(sceUltUlthreadCreate(&thread, "parked", Parked, 0x5A, nullptr, 0, &runtime, nullptr, 0) == Ok);
+    while (!gStarted.load(std::memory_order_acquire)) std::this_thread::yield();
+
+    const int outstanding = sceUltFinalize();
+    gRelease.store(true, std::memory_order_release);
+    std::int32_t status = 0;
+    const int joined = sceUltUlthreadJoin(&thread, &status);
+
+    CHECK(outstanding == Busy);
+    CHECK(joined == Ok && status == 0x5A);
+    CHECK(sceUltUlthreadJoin(&thread, &status) == State);
+    CHECK(sceUltFinalize() == Ok);
+}
+
+static void Validation() {
+    Runtime runtime;
+    Thread thread;
+    gStarted.store(false, std::memory_order_release);
+    gRelease.store(false, std::memory_order_release);
+    CHECK(sceUltUlthreadRuntimeCreate(nullptr, "runtime", 1, 1, nullptr, nullptr, 0) == Null);
+    CHECK(sceUltUlthreadRuntimeCreate(&runtime, "runtime", 1, 1, nullptr, nullptr, 0) == Ok);
+    CHECK(sceUltUlthreadCreate(nullptr, "thread", Immediate, 0, nullptr, 0, &runtime, nullptr, 0) == Null);
+    CHECK(sceUltUlthreadCreate(&thread, "thread", nullptr, 0, nullptr, 0, &runtime, nullptr, 0) == Null);
+    CHECK(sceUltUlthreadCreate(&thread, "thread", Immediate, 0, nullptr, 0, nullptr, nullptr, 0) == Null);
+    CHECK(sceUltUlthreadJoin(nullptr, nullptr) == Null);
+    CHECK(sceUltUlthreadJoin(&thread, nullptr) == State);
+
+    CHECK(sceUltUlthreadCreate(&thread, "thread", Immediate, 0x27, nullptr, 0, &runtime, nullptr, 0) == Ok);
+    CHECK(sceUltUlthreadCreate(&thread, "thread", Immediate, 0x27, nullptr, 0, &runtime, nullptr, 0) == State);
+    CHECK(sceUltFinalize() == Busy);
+    std::int32_t status = 0;
+    CHECK(sceUltUlthreadJoin(&thread, &status) == Ok && status == 0x27);
+    CHECK(sceUltFinalize() == Ok);
+    CHECK(sceUltUlthreadRuntimeGetWorkAreaSize(1, 1) == 256u + 16u * 1024u);
+}
+
+int main(int argc, char** argv) {
+    CHECK(argc == 2);
+    CHECK(sceUltInitialize() == Ok);
+    if (std::strcmp(argv[1], "outstanding") == 0) Outstanding();
+    else if (std::strcmp(argv[1], "validation") == 0) Validation();
+    else CHECK(false);
+}