Jonathan Majors Sings 'Happy Birthday' To TMZ Tour Bus

Jonathan Majors isn't staying out of the spotlight amid his legal woes -- just ask the riders of the TMZ Tour ... who caught the dude in the flesh, and heard him flex his pipes!

Check out this video we got of JM from last week, when our tour guide spotted him driving next to our bus in the middle of a tour of Los Angeles. He was nice enough to chat with us and our guests for a bit ... and once he learned it was someone's birthday, he started singing.

Looks like Jon was riding solo here with his pup, but nonetheless ... he gave it his all.

This was actually the second time in a week that TMZ caught up with Jonathan. Prior to this, the former Marvel actor revealed to us that he wasn't sweatin' his ex Grace Jabbari's lawsuit against him -- instead, he's working up a sweat to stay fit for what sounds like a new role!

Grace -- who testified against him in his criminal trial in December -- is suing him, claiming he assaulted her over the course of their relationship and defamed her as well. This comes after Jonathan was convicted in his criminal trial at the end of last year.

He's currently awaiting sentencing ... but his legal team is working on having the conviction tossed out. Jonathan told us he wasn't surprised by Grace's lawsuit -- but is trying to move past it all.