diff --git a/dll/kernel32/fileapi.cpp b/dll/kernel32/fileapi.cpp
index b65ce90..31b5083 100644
--- a/dll/kernel32/fileapi.cpp
+++ b/dll/kernel32/fileapi.cpp
@@ -1748,7 +1748,7 @@ UINT WINAPI GetTempFileNameA(LPCSTR lpPathName, LPCSTR lpPrefixString, UINT uUni
 	HOST_CONTEXT_GUARD();
 	DEBUG_LOG("GetTempFileNameA(%s, %s, %u)\n", lpPathName ? lpPathName : "(null)",
 			  lpPrefixString ? lpPrefixString : "(null)", uUnique);
-	if (!lpPathName || !lpPrefixString || !lpTempFileName) {
+	if (!lpPathName || !lpTempFileName) {
 		setLastError(ERROR_INVALID_PARAMETER);
 		return 0;
 	}
@@ -1757,9 +1757,12 @@ UINT WINAPI GetTempFileNameA(LPCSTR lpPathName, LPCSTR lpPrefixString, UINT uUni
 		return 0;
 	}
 
+	// A null prefix is treated as an empty string, matching Windows/Wine.
+	const char *prefix = lpPrefixString ? lpPrefixString : "";
+
 	auto makeTempFileName = [&](UINT unique) {
 		char filename[12];
-		snprintf(filename, sizeof(filename), "%.3s%04X.TMP", lpPrefixString, unique & 0xFFFF);
+		snprintf(filename, sizeof(filename), "%.3s%04X.TMP", prefix, unique & 0xFFFF);
 
 		std::string path = lpPathName;
 		if (!path.empty() && path.back() != '\\' && path.back() != '/') {
@@ -1810,7 +1813,27 @@ UINT WINAPI GetTempFileNameA(LPCSTR lpPathName, LPCSTR lpPrefixString, UINT uUni
 		lpTempFileName[MAX_PATH - 1] = '\0';
 		DEBUG_LOG(" -> %s\n", lpTempFileName);
 	}
-	return uUnique;
+	return uUnique & 0xFFFF;
+}
+
+UINT WINAPI GetTempFileNameW(LPCWSTR lpPathName, LPCWSTR lpPrefixString, UINT uUnique, LPWSTR lpTempFileName) {
+	HOST_CONTEXT_GUARD();
+	DEBUG_LOG("GetTempFileNameW -> ");
+	if (!lpPathName || !lpTempFileName) {
+		setLastError(ERROR_INVALID_PARAMETER);
+		return 0;
+	}
+	std::string pathName = wideStringToString(lpPathName);
+	// A null prefix is treated as an empty string, matching Windows/Wine.
+	std::string prefixString = lpPrefixString ? wideStringToString(lpPrefixString) : std::string();
+	char tempFileName[MAX_PATH];
+	UINT result = GetTempFileNameA(pathName.c_str(), prefixString.c_str(), uUnique, tempFileName);
+	if (result == 0) {
+		return 0;
+	}
+	auto wide = stringToWideString(tempFileName);
+	wstrncpy(lpTempFileName, wide.data(), wstrlen(wide.data()) + 1);
+	return result;
 }
 
 DWORD WINAPI GetTempPathA(DWORD nBufferLength, LPSTR lpBuffer) {
diff --git a/dll/kernel32/fileapi.h b/dll/kernel32/fileapi.h
index 7531963..0e1d443 100644
--- a/dll/kernel32/fileapi.h
+++ b/dll/kernel32/fileapi.h
@@ -61,6 +61,7 @@ DWORD WINAPI GetFullPathNameW(LPCWSTR lpFileName, DWORD nBufferLength, LPWSTR lp
 DWORD WINAPI GetShortPathNameA(LPCSTR lpszLongPath, LPSTR lpszShortPath, DWORD cchBuffer);
 DWORD WINAPI GetShortPathNameW(LPCWSTR lpszLongPath, LPWSTR lpszShortPath, DWORD cchBuffer);
 UINT WINAPI GetTempFileNameA(LPCSTR lpPathName, LPCSTR lpPrefixString, UINT uUnique, LPSTR lpTempFileName);
+UINT WINAPI GetTempFileNameW(LPCWSTR lpPathName, LPCWSTR lpPrefixString, UINT uUnique, LPWSTR lpTempFileName);
 DWORD WINAPI GetTempPathA(DWORD nBufferLength, LPSTR lpBuffer);
 HANDLE WINAPI FindFirstFileA(LPCSTR lpFileName, LPWIN32_FIND_DATAA lpFindFileData);
 HANDLE WINAPI FindFirstFileW(LPCWSTR lpFileName, LPWIN32_FIND_DATAW lpFindFileData);
diff --git a/test/test_gettempfilename.c b/test/test_gettempfilename.c
index 776a4d6..85a1e25 100644
--- a/test/test_gettempfilename.c
+++ b/test/test_gettempfilename.c
@@ -2,6 +2,7 @@
 
 #include <stdio.h>
 #include <string.h>
+#include <wchar.h>
 #include <windows.h>
 
 static const char *basename_of(const char *path) {
@@ -9,15 +10,20 @@ static const char *basename_of(const char *path) {
 	return slash ? slash + 1 : path;
 }
 
+static const WCHAR *basename_of_w(const WCHAR *path) {
+	const WCHAR *slash = wcsrchr(path, L'\\');
+	return slash ? slash + 1 : path;
+}
+
 static void expect_fixed_unique_name(const char *temp_path) {
 	char temp_file[MAX_PATH];
 
 	UINT result = GetTempFileNameA(temp_path, "wboX", 0x12345, temp_file);
-	TEST_CHECK_EQ(0x12345, result);
+	TEST_CHECK_EQ(0x2345, result);
 	DeleteFileA(temp_file);
 
 	result = GetTempFileNameA(temp_path, "wboX", 0x12345, temp_file);
-	TEST_CHECK_EQ(0x12345, result);
+	TEST_CHECK_EQ(0x2345, result);
 	TEST_CHECK_STR_EQ("wbo2345.TMP", basename_of(temp_file));
 	TEST_CHECK_EQ(INVALID_FILE_ATTRIBUTES, GetFileAttributesA(temp_file));
 }
@@ -39,6 +45,64 @@ static void expect_created_file_reopens_like_link(const char *temp_path) {
 	TEST_CHECK(CloseHandle(handle));
 }
 
+static void expect_null_prefix_is_empty(const char *temp_path) {
+	char temp_file[MAX_PATH];
+
+	UINT result = GetTempFileNameA(temp_path, NULL, 0x12345, temp_file);
+	TEST_CHECK_EQ(0x2345, result);
+	TEST_CHECK_STR_EQ("2345.TMP", basename_of(temp_file));
+}
+
+static void expect_fixed_unique_name_w(const WCHAR *temp_path) {
+	WCHAR temp_file[MAX_PATH];
+
+	UINT result = GetTempFileNameW(temp_path, L"wboX", 0x12345, temp_file);
+	TEST_CHECK_EQ(0x2345, result);
+	DeleteFileW(temp_file);
+
+	result = GetTempFileNameW(temp_path, L"wboX", 0x12345, temp_file);
+	TEST_CHECK_EQ(0x2345, result);
+	TEST_CHECK(wcscmp(L"wbo2345.TMP", basename_of_w(temp_file)) == 0);
+	TEST_CHECK_EQ(INVALID_FILE_ATTRIBUTES, GetFileAttributesW(temp_file));
+}
+
+static void expect_created_file_reopens_like_link_w(const WCHAR *temp_path) {
+	WCHAR temp_file[MAX_PATH];
+
+	UINT result = GetTempFileNameW(temp_path, L"GDI32", 0, temp_file);
+	TEST_CHECK(result != 0);
+	TEST_CHECK_EQ(11, wcslen(basename_of_w(temp_file)));
+	TEST_CHECK_EQ(0, wcsncmp(L"GDI", basename_of_w(temp_file), 3));
+	TEST_CHECK(wcscmp(L".TMP", basename_of_w(temp_file) + 7) == 0);
+	TEST_CHECK(GetFileAttributesW(temp_file) != INVALID_FILE_ATTRIBUTES);
+
+	HANDLE handle = CreateFileW(temp_file, GENERIC_READ | GENERIC_WRITE, 0, NULL, OPEN_EXISTING,
+								FILE_ATTRIBUTE_NORMAL | FILE_FLAG_DELETE_ON_CLOSE, NULL);
+	TEST_CHECK_MSG(handle != INVALID_HANDLE_VALUE, "CreateFileW(temp, OPEN_EXISTING|DELETE_ON_CLOSE) failed: %lu",
+				   GetLastError());
+	TEST_CHECK(CloseHandle(handle));
+}
+
+static void expect_null_prefix_is_empty_w(const WCHAR *temp_path) {
+	WCHAR temp_file[MAX_PATH];
+
+	UINT result = GetTempFileNameW(temp_path, NULL, 0x12345, temp_file);
+	TEST_CHECK_EQ(0x2345, result);
+	TEST_CHECK(wcscmp(L"2345.TMP", basename_of_w(temp_file)) == 0);
+}
+
+static void expect_invalid_parameters_w(const WCHAR *temp_path) {
+	WCHAR temp_file[MAX_PATH];
+
+	SetLastError(0);
+	TEST_CHECK_EQ(0, GetTempFileNameW(NULL, L"wbo", 0, temp_file));
+	TEST_CHECK_EQ(ERROR_INVALID_PARAMETER, GetLastError());
+
+	SetLastError(0);
+	TEST_CHECK_EQ(0, GetTempFileNameW(temp_path, L"wbo", 0, NULL));
+	TEST_CHECK_EQ(ERROR_INVALID_PARAMETER, GetLastError());
+}
+
 int main(void) {
 	char temp_path[MAX_PATH];
 	DWORD len = GetTempPathA(sizeof(temp_path), temp_path);
@@ -46,6 +110,16 @@ int main(void) {
 
 	expect_fixed_unique_name(temp_path);
 	expect_created_file_reopens_like_link(temp_path);
+	expect_null_prefix_is_empty(temp_path);
+
+	WCHAR temp_path_w[MAX_PATH];
+	int wlen = MultiByteToWideChar(CP_ACP, 0, temp_path, -1, temp_path_w, MAX_PATH);
+	TEST_CHECK(wlen > 0);
+
+	expect_fixed_unique_name_w(temp_path_w);
+	expect_created_file_reopens_like_link_w(temp_path_w);
+	expect_null_prefix_is_empty_w(temp_path_w);
+	expect_invalid_parameters_w(temp_path_w);
 
 	return 0;
 }