TMZ Sports has learned ... the Heisman Trophy winner wanted to do something special after inking his massive, $36 million rookie deal, so he hit up celeb jeweler Leo Frost to whip up the one-of-a-kind piece.

We're told the awesome #9 pendant is made of 18k white gold and VVS diamonds ... sitting on an 18k white gold prong-set Cuban chain with VVS diamonds.

In other words, the rook's got taste!!

For those who paid close attention to our pre-draft jewelry roundup back in February, this should come as no surprise. As Leo told Burrow at the time, "After the draft, I'm thinking we make this a fat ass piece for you, #9? A fat #9."

Leo delivered on his promise ... flying to Cincinnati this week to hand over the final product ... and Joe loved it!!

It's been one crazy year for the Ohio boy -- he won a natty, won the Heisman, helped raise $500k for his hometown food bank, and went #1 overall in the Draft.