Justin Bieber -- I Wore $20K On My NECK at Mayweather Fight

Looking like he wandered off the set of "Goodfellas," Justin Bieber rocked a golden diamond-encrusted fish around his neck at the Mayweather fight on Saturday -- and TMZ has learned, the thing cost $20,000!!!

Sources tell us, the 14k gold "koi" fish -- designed by celeb jeweler Ben Baller -- boasts 14 carats of VVS diamonds all over (supposedly fancier than normal non-VVS diamonds). Even the chain is covered in 12 carats of diamonds.

it's unclear if the necklace was a gift or if Bieber paid for it himself.

FYI -- Koi fish represent good luck. Not like Mayweather needed it.