Stefon Diggs isn't just nice on the football field, he's also a problem on the hardwood -- 'cause the NFL star just flexed some insane skills during a recent hoops sesh ... including an impressive dunk!!

The Buffalo Bills receiver was shootin' buckets at the famous Summit NYC gym with famous NBA skills trainer Chris Brickley on Thursday ... and was captured on video doin' what he calls "lab work."

In a clip of the workout, the 3-time Pro Bowler goes off with a couple of nice step-backs, a reverse layup, and a turnaround fadeaway.

But, what was most impressive ... Diggs also displayed his range and hops by hitting a shot behind the arc and throwing down a massive dunk!

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