Chris Pratt Sings Garth Brooks and Johnny Cash Songs in Nashville

Chris Pratt is going country ... hopping onstage to belt out some Johnny Cash and Garth Brooks tunes during a couple of surprise performances in Nashville. 

The 'Guardians of the Galaxy' star hit up a couple honky tonks Thursday night ... grabbing the mic at Tootsies to belt out Garth Brooks' "Papa Loved Mama." Ya can't say Chris held back -- he went all in for the performance, even if he was a little loose with the lyrics.

But, one stop wasn't enough for CP on this night. He also went next door to test his voice onstage at Robert's Western World for a rendition of Johnny Cash's "Folsom Prison Blues." 

Chris belted out the country music classic alongside musician Sarah Gayle Meech ... and as you can see, they rocked the house with a full band behind them!

Folks who were there say Chris totally made their night ... and Katherine Schwarzenegger's gotta be pretty proud of her husband's pipes. Or, at least his energy.

Johnny was famous for always wearing black ... but Chris went a different route and kept it low key with a denim button down and a white hat. Johnny would approve of the actor's black pants, though.