Odell Beckham had one of the most expensive outfits at the Met Gala ... and it's all thanks to his incredible $650k diamond chain!!
TMZ Sports spoke with OBJ's go-to guy, Gabriel the Jeweler ... who tells us the wide receiver copped the ice right before Monday's big event.
Beckham donned a green velvet hoodie for the Gala ... accenting it with the massive chain, rings and a grill.
The chain is truly one of a kind -- we're told it's made with 300 carats of round diamonds, 200 carats of emerald-cut diamonds and 2 1/2 kilos of gold.