Timothée Chalamet might be a "Complete Unknown" in theaters ... but he certainly wasn't one at the Knicks game Monday -- 'cause Karl-Anthony Towns took some time out of his postgame routine to gift him a jersey!!

The cool scene all unfolded just after KAT and New York beat up the Memphis Grizzlies at Madison Square Garden.

The 29-year-old center walked over to the actor -- who had some sweet courtside seats for the contest -- stripped off his top, signed it and handed it over. The two then posed for a grip of pictures.

Chalamet was clearly a fan of the gesture -- he repeatedly thanked Towns, calling him his "dawg" in the process.

Timmy's response, of course, should surprise nobody -- he's been a diehard Knicks supporter since he was a kid ... even once winning a contest for tickets back before he ever wowed audiences on the silver screen.

It was a nice cap to a solid weekend for Chalamet -- as he also crushed it on "Saturday Night Live" just two days prior to the game.