Peyton Manning -- I Shot a Man In Reno ... Just to Watch Him Die

Peyton Manning walked the line with a bigtime country singer this weekend -- belting out a Johnny Cash classic at a charity event ... and it was all caught on video.

The Denver Broncos quarterback was at his Celebration of Caring Gala in Indianapolis on Saturday -- when country star Jake Owen called him on stage to sing "Folsom Prison Blues."

Manning sang like a champ -- and even got an ovation at the end.