The Weeknd, Jimmy Fallon Crash Fordham Seniors' Graduation Bash

Jimmy Fallon and The Weeknd gave a group of Fordham University seniors a surprise they'll never forget -- they crashed their graduation bash at the Lincoln Center campus in New York City!

Check out the video ... the graduating seniors were absolutely gobsmacked when they saw the Grammy-winner walk through the doors and start performing his hit song "Blinding Lights" Thursday -- inspiring an impromptu sing-along.

TMZ is told he even did an encore, singing "Can’t Feel My Face" with fans as well.

Jimmy was on DJ duty for the sweet surprise, and we're told both students and faculty lost their minds when he took over the DJ booth.

The wild moment will be highlighted on Thursday's "The Tonight Show" episode, our sources say ... on which The Weeknd is scheduled to join JF for his first-ever late-night interview.

It's been a busy few days for The Weeknd ... remember -- he took over the DJ booth himself at the lively after-party for his new film, "Hurry Up Tomorrow" just the other night. He was spotted groovin' with his co-star Jenna Ortega and even holding hands with his better half, Simi Khadra.