Taylor Swift's boyfriend flew in to the party town this week to help Kittle and Greg Olsen throw a massive event for their "Tight End University" ... and following some afternoon activities with the guys, he blew off a little steam with the duo in the evening.

The three hit the stage at the Brooklyn Bowl -- and got the crowd ready for a special "Tight Ends & Friends" show that featured performances from Jelly Roll, Jon Pardi and more.

Kittle initially got the people going -- before Kelce hopped on the mic ... and told the packed house, "Let's f***ing rock!"

Kelce and Kittle were then seen slamming some beers on the side of the stage.

The future Hall of Famer is expected to remain in town for a few more days -- as TEU has a few other events planned for the week.

It's, of course, just the latest fun stop for Kelce since he and the Chiefs won Super Bowl LVIII ... he's been spotted all over the globe, most recently in Kansas City where he got his latest title bling.