The "Riverdale" star -- who plays Archie Andrews on the hit CW show -- hopped onstage with Canadian rock band The Steadies Saturday night at Guilt & Co. in Vancouver ... and tore it up for them on the electric guitar.

We are told KJ goes to the small club a lot to enjoy the live music -- because "Riverdale" films nearby -- but it was the first time he had ever performed there. Apa led the funky jams for the band out of Saskatoon for about 35 minutes ... and plans on joining them again.

Our sources say KJ might even play on The Steadies' next album ... and he has made it known, his real dream is to be a musician.

It's life imitating art ... or vice versa. Either way -- rock on, Archie!