The bronze sword, which has an octagonal hilt, is thought to be from the late 14th Century BC.
Its condition is so good "it almost still shines", Bavaria's State Office for Monument Protection (BLfD) says as quoted by BBC.
The grave contains the bones of a man, woman and boy, and other bronze items, the British media reported.
The BLfD says manufacturing such a sword was complicated, as the hilt was cast on to the blade.