Ariana Grande wrote some songs about Ricky ... and now he's at her show!

That's right -- the pop star's ex Ricky Alvarez attended her "Eternal Sunshine" tour Friday night in Austin, Texas ... after we told you they still hang out as friends. Check out the pic -- he looks thrilled to be supporting her from the floor for her birthday show.

Ariana turned 33 on Friday, and her backup dancers took the opportunity to surprise her with a cake on stage last night.

ICYMI, Ricky danced in Ari's "Honeymoon" tour over a decade ago and also performed in some of her music videos ... they dated for a bit, and he even got a shout-out in her 2019 hit, "thank u, next."

She playfully changed the lyrics to Ricky's shout-out in the song twice recently -- from "Wrote some songs about Ricky / Now I listen and laugh" to "... they still kinda slap" and "... he still got my back." Ariana's pops was even spotted giving Ricky a handshake Friday night during the lyric change!

But don't get too excited -- sources tell TMZ the pair are just friends, and she invited a few out to Austin to celebrate her birthday.

As you know, the "Wicked" star is single right now after she and her costar Ethan Slater split after nearly 3 years of dating.