John Mayer may be in a whole heap of trouble with GF, Jessica Simpson today. Why? 'Cause he kissed another girl on Saturday night -- and we have the video to prove it!

Here's what happened: John was reading signs that his fans made for his concert when he came across one that said, "Sweet 16 and never been kissed. Wanna be my first?" Apparently, the answer was yes 'cause he jumped off the stage and gave the girl a kiss -- on the lips!