You never know who or what you're going to see on the TMZ Celebrity Tour ... and today it was Meghan Trainor filming a music video for a new song ... and she got on the bus with cameras rolling!!!

Our tour bus was cruising through Beverly Hills when we spotted Meghan on the corner of Beverly Drive filming a music vid for her upcoming single, "Nope, I Still Don't Care."

Meghan smiled and waved as we leaned out the window and told her someone on the bus was celebrating a birthday, and she sent love and wishes ... then went one step further.

As we drove away to let Meghan do her thing, someone from her team ran after our bus and asked us to turn around ... and we obliged.

Meghan hopped on the bus with cameras in tow and took selfies with everyone on board ... and she even filmed some content for her music video.

Check out the clip ... and get your TMZ Celebrity Tour tickets now!!!