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 and the guys collabed 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 ... and it's worth around $20k.

The back of the pendant also features Jaylen's jersey number, 7, and his name.

The video is awesome ... Ferg and Big Will were there to celebrate with Brown ... and the dude had a BIG smile on his face when they presented him with the present.

Happy birthday!!