Taylor Swift -- Sued By Radio DJ ... I Never Grabbed Her Ass!!

Taylor Swift's ass is in the middle of a lawsuit -- a former Denver radio DJ says he got canned after her security accused him of grabbing Taylor's cheeks.

David Mueller says he was attending a Swift concert in Denver in 2013 and was invited backstage. According to The Denver Post, Mueller says he and his GF took a photo with Swift and then left to enjoy the show.

Mueller says he later ran into a coworker who was bragging about putting his arms around Taylor and grabbing her butt during his photo op with her.

Here's where it gets interesting ... Mueller says in a lawsuit, he was approached by one of Swift's security guards who accused him of grabbing Taylor's tush and kicked him out of the arena. Mueller says he was fired from his country music station because of the incident.