Zac Efron's officially been put on notice ... Simone Biles' BF fired off the warning shot.
Arthur Nory Mariano -- also a stellar gymnast reppin' Brazil at the Olympics -- just tweeted at Zac to back off his gold medal winning GF. He's joking ... we think.
Arthur's an Olympic medalist and Zac's packing heat ... a very nice 12-pack for any party.
The question is ...