Jennifer Lopez heard fans were "obsessed" ... so she showed up to see it for herself.

JLo made a surprise appearance at Prime Video's Obsessed Fest over the weekend ... popping out to thunderous cheers before introducing the cast of the streamer's hit series, "Off Campus." TMZ caught video of the moment -- and you've gotta check it out to see the fans absolutely lose it as she hits the stage.

The singer and actress walked out to her own 2011 smash, "On the Floor" ... and also performed "Everything's Fine" as she soaked up the love from the stunned crowd who was no doubt getting loud.

This mini-concert comes on the heels of JLo being spotted in the audience of Ariana Grande's "Eternal Sunshine" tour stop in L.A. last weekend. Jenny from The Block was vibing out to Ari's hits ... and maybe even enjoying having the spotlight off her for a moment.

While this may have been an "Off Campus" fan event, it was a JLo fan event to us!!!