Not officially announced by MTV, the 20-year-old offered an emotionally charged performance of an unreleased song midway through Sunday's telecast.
Her surprise performance began with video clips of last year's infamous stage-crashing, and though West was never officially named, the song's lyrics all but made overt references to him. "Some things you can't speak of," she sang, "but tonight you live it all again."
Rising from her chair – in a Gothic living room tableau – Swift walked, barefoot, into the center of the stage as the song swelled. "You have new Septembers," she sang. "Everyone one of us has messed up, too."
Seemingly reassuring herself, Swift sang, "It's all right, just wait and see. Your string of lights is still bright to me. You're still an innocent."
When the anthem concluded, the singer was rewarded with a standing ovation. West, who is scheduled to close Sunday's telecast, was not seen during the performance.
The unnamed song is set to appear on Swift's third album, Speak Now, due Oct. 25.