The G.O.A.T. joined the Bruins for an impromptu throwing sesh Wednesday morning ... and seems the Patriots QB enjoyed it -- he smiled and took pics with all the guys afterward.

Of course, this is becoming an annual thing for the NFL superstar ... just 12 months ago, Brady was in town and ran an entire workout with the Bruins' offense.

This week's sesh seemed much less formal ... but UCLA studs like Chase Cota were still jacked up for the day anyway.

"Got to work with THE GOAT today," the Bruins receiver said ... "What an opportunity, go Bruins!"

By the way ... UCLA continued the tradition of throwing some shade at USC for the star-studded workout -- tagging their pictures once again with, "#OnlyAtUCLA."