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Ariana Grande walks into a bar is how you'd start a good joke, but this time it's real and ends with our girl locking lips with a mystery man who might not even be all that famous.

The singer was spotted out late Saturday night at a place called Bar Louie in Northridge, CA -- which is in the San Fernando Valley ... way on the outskirts of central L.A.

We're told she rolled into the gastropub around 1 AM with a crew of friends in tow, including this gent you see here right next to her in the booth. They popped in, sat down for a bit, and besides the water we're told AG wanted ... she was just all in on this guy's face.

As for who this fella might be -- it's unclear. We're told he wasn't instantly recognizable though ... which means he's not on Ariana's level (fame-wise, anyway).

Now, eyewitnesses tell us the group only stayed for around 30 minutes, and while some of her friends ordered wine and other drinks ... we're told Ariana wasn't boozing.

She was, however, in a chatty mood. Our sources tell us there was a DJ nearby who was actually playing Ariana's song, "God is a Woman," right around the same time she was there. We're told AG asked the guy to switch up the tunes ... she didn't wanna hear her own song.

A couple interesting things about this smooching sesh -- for one, it doesn't look like Social House singer Mikey Foster, who's been romantically linked to Ms. Grande of late ... even though Ariana's brother clarified there was nothing going on. They hang out a lot, that's for darn sure.

The second thing ... Ariana is apparently a regular in the Northridge area. She was interviewed there for a Vogue article in 2019 and has been seen out and about.