Best birthday EVER?!?!
Boston Celtics star Jaylen Brown is fresh off a brand new, $115 MILLION deal ... and if that's not sweet enough, the baller got a sick, $20k chain from his good buddy, A$AP Ferg!!
TMZ Sports has learned ... the "Floor Seats" rapper wanted to do something extra special for his friend's birthday, so he hit up Big Will at WS Jewelry to make him a one-of-a-kind piece.
We're told the piece comes with 15 carats of natural green and white diamonds shaped in Ferg's signature logo .