diff --git a/src/port/win32pread.c b/src/port/win32pread.c
index e1a066fdbe..2d022e6d37 100644
--- a/src/port/win32pread.c
+++ b/src/port/win32pread.c
@@ -30,6 +30,9 @@ pg_pread(int fd, void *buf, size_t size, off_t offset)
 		return -1;
 	}
 
+	/* Avoid overflowing DWORD. */
+	size = Min(size, 1024 * 1024 * 1024);
+
 	/* Note that this changes the file position, despite not using it. */
 	overlapped.Offset = offset;
 	if (!ReadFile(handle, buf, size, &result, &overlapped))
diff --git a/src/port/win32pwrite.c b/src/port/win32pwrite.c
index c54bf041bf..b37bb2f92e 100644
--- a/src/port/win32pwrite.c
+++ b/src/port/win32pwrite.c
@@ -30,6 +30,9 @@ pg_pwrite(int fd, const void *buf, size_t size, off_t offset)
 		return -1;
 	}
 
+	/* Avoid overflowing DWORD. */
+	size = Min(size, 1024 * 1024 * 1024);
+
 	/* Note that this changes the file position, despite not using it. */
 	overlapped.Offset = offset;
 	if (!WriteFile(handle, buf, size, &result, &overlapped))