Sydney Sweeney and Scooter Braun look like a seriously fun couple ... because we got video of them singing karaoke ... and their duet had the whole joint rocking!!!

Check out this footage of Sydney and Scooter doing karaoke in Ventura, CA ... they each had a mic as they belted out Neil Diamond's classic hit "Sweet Caroline" ... and they brought the house down. So good, so good, so good.

We also got video of Sydney singing "Gimme! Gimme! Gimme" by ABBA ... and Scooter may need to think about signing his girlfriend to a record deal. He certainly knows talent when he sees it!!!

It seems obvious from the videos, but Sydney and Scooter look like they're having a ton of fun together ... and she's got a lot to be happy about.

The "Euphoria" star recently released her lingerie line, SYRN, and it's clearly been a success because Kim Kardashian also dropped a SKIMS line this week ... so it seems Kim's keeping an eye on the competition.