Ricky Alvarez was front and center supporting girlfriend Ariana Grande over the weekend ... showing up at her concert and looking like he knew every word.

Check out the video ... Ricky is seen dancing and singing along to Ariana's song "Imperfect for You," smiling as she performs for the packed crowd Saturday night at Barclays Center in Brooklyn.

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His appearance comes after TMZ revealed Ariana and Ricky have rekindled their romance following her split from Ethan Slater.

As we previously reported, the two were recently spotted packing on the PDA during a stroll through New York City.

TMZ obtained photos showing Ariana wrapping an arm around Ricky while he grabbed a handful of her backside before the pair took a romantic walk through Central Park with her dog, Toulouse.

Ariana and Ricky famously dated years ago before calling it quits, but it appears they've found their way back to each other.