Taylor Swift & Travis Kelce Sing 'You Belong With Me' at Super Bowl After-Party

Taylor Swift and Travis Kelce are on top of the world after a photo finish at the Super Bowl -- and they celebrated their love in epic fashion afterward, including with a song.

The couple hit up a private party at Zouk inside Resorts World Las Vegas Sunday night -- where the Chainsmokers were handling music duties for a packed club -- and one of the highlights was Tay Tay and TK singing one of her most famous songs to one another from afar.

Of course, we're talking about "You Belong With Me" ... which the DJ duo started playing/mixing into an EDM-inspired track -- and which caused Travis to jump into their booth immediately.

Check it out ... the dude was more than happy to start belting out the well-known tune, and he even turned to Taylor -- who was out in the crowd behind him -- to serenade her from where he was at. She was singing back to him too BTW ... they were singing to each other.

It was pretty sweet ... and they actually kept the lovey-dovey vibes going by dancing together to the same track later on. And yes, there was a lot of smooching going on between them as well. Taylor and Travis were locking lips all night, and tied at the hip coming and going.

Other highlights from their wild night out ... Post Malone came onstage to perform, which is timely considering the guy is one of the only features on her upcoming album, "The Tortured Poets Department."

Tyga and Ludacris also performed at Zouk -- and the place was absolutely bumping the entire time.

Taylor and Travis didn't just hit up Zouk, BTW -- although, they did seem to start the night there. They were also spotted at XS Las Vegas a little later... smooching there as well.

Indeed, it's the real-life love story everyone's been watching ... and it came to a fairy tale conclusion this weekend. Like we said, TS and TK are head over heels, and it shows.

The obvious question now is ... what happens between them now??? Love ... marriage?