Floyd Mayweather spent an ungodly amount of money on jewelry during an L.A. shopping spree last week ... dropping $5.3 MILLION on new chains and bracelets, TMZ Sports has learned.

The champ hit up jeweler to the stars Peter Marco at his shop in Beverly Hills on Nov. 14 ... and went HAM. Here's the breakdown:

-- One 30-carat pear-shaped, fancy yellow VVS ring for $2.3 MILLION.

-- One 3-4.5 ct. diamond bracelet for another $2.5 MILLION

-- One 2-pound yellow gold Cuban necklace and 3 watches for a combined $500k

Add all that up ... and you got a whoppin' $5.3 mil.

How'd Mayweather pay for all of the stuff? He wrote checks! (Hey, can't pay for everything in cash!)

Of course, Floyd ain't shy about his insane purchases ... remember his $18 MILLION watch ... and his $2 MILLION car fleet ... and oh yeah, his $10 MILLION ring and chain combo.

Makes ya kinda wish you became an undefeated international boxing superstar, huh?

Floyd is still lining up big money events for himself -- he claims he'll make a fortune for fighting Tenshin Nasukawa in Japan on New Year's Eve.

Plus, Floyd told us he'll make a 9-FIGURE payday if he can come to terms with Khabib Nurmagomedov for a boxing superfight in 2019.