Sean Kaufman and Avantika are featured in an upcoming music video for WHATMORE ... and only TMZ has behind-the-scenes footage from the production.

"The Summer I Turned Pretty" hunk and the "Mean Girls" star linked up recently in New York City to film the music vid for WHATMORE's track, "2000s Pop Punk Rnb" ... and filming locations included a subway car, the MET and the New York Public Library.

Check out the footage, it looks like Sean and Avantika are having a lot of fun.

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It's not a random collab ... we're told Sean and Avantika are friends with the band.

The full music video is dropping soon ... stay tuned!!!