Taylor Swift just laid down the law ... nobody grabs her ass and gets away with it ... because she just countersued a radio DJ who claims she falsified a story about getting groped.
According to the docs ... Swift says she's 100% sure David Mueller lifted her skirt and grabbed her butt in 2013 at a backstage photo op for her Denver concert.
As we previously reported ... Mueller initially sued Swift ... claiming he was fired from his radio gig after the singer claimed the radio jock groped her.
Mueller now claims the assault was carried out by another guy ... but in her docs, Taylor insists Mueller was the one who got handsy, leaving the singer, "surprised, upset, offended, and alarmed."
Taylor's asking for damages to be assessed by a jury, and says she'll be donating any money she wins to women's charities.Follow Me On Twitter & Instagram: @effiongeton