diff --git a/platforms/posix/src/px4/windows/posix/fs/mman.cpp b/platforms/posix/src/px4/windows/posix/fs/mman.cpp
index c817f7aa97..38d083996f 100644
--- a/platforms/posix/src/px4/windows/posix/fs/mman.cpp
+++ b/platforms/posix/src/px4/windows/posix/fs/mman.cpp
@@ -60,6 +60,14 @@ static DWORD mman_page_protect(int prot)
 
 extern "C" void *mmap(void *, size_t length, int prot, int flags, int fd, off_t offset)
 {
+	/* MAP_FIXED is rejected because the Windows backend (VirtualAlloc /
+	 * MapViewOfFile) cannot safely replace existing mappings at a caller
+	 * supplied address. The shim header documents this contract. */
+	if (flags & MAP_FIXED) {
+		errno = ENOTSUP;
+		return MAP_FAILED;
+	}
+
 	const bool anon = (flags & (MAP_ANON | MAP_ANONYMOUS)) != 0;
 
 	if (anon || fd < 0) {