Lady Gaga's Boyfriend Grabs Her Butt -- Just a Pinch... [Video]

Lady Gaga was the victim of a drive-by butt pinching Tuesday afternoon in New York ... but don't worry, the assailant was none other than her boyfriend, Taylor Kinney.

Gaga was leaving her apartment in midtown Manhattan when she decided to stop and sign a few autographs for fans. With her back turned, Kinney came out behind Gaga and made his move ... catching her ass totally by surprise with the ole pinch-and-move.