Forget "That '70s Show" -- Laura Prepon had herself a karaoke show in Nashville over the weekend ... fearlessly hopping up on stage and belting out Weezer's "Say It Ain't So."

Check it out ... the actress grabbed the mic at The Lipstick Lounge and looked like she was having a blast as she worked the stage ... encouraging the audience to sing along and sharing her mic with the crowd. She appeared to be in the company of some friends, too.

An eyewitness named Sav tells TMZ the bell-bottomed beauty seemed "so down to earth" and "just like a regular girl hanging with her friends" as she commanded the stage. Sav adds Laura had fantastic energy and was "interacting with everyone."

Sav even admits she didn't realize Laura was actually Alex Voss from "Orange Is the New Black" -- which Laura starred on for 7 seasons beginning in 2013. Our eyewitness says she even told Laura she looks like the spitting image of her 'OITNB' character ... and that she playfully "smirked" in response.

Sav gushes it was "such a cool moment to be able to catch" and the crowd loved Laura's "iconic" song choice.

If her acting career dries up ... looks like she feels just as comfortable with a mic in hand!