"They're over," a source tells PEOPLE about the short-lived relationship. "It ended last month."
No reason was given for a split, and the stars reps didn't have immediate comment, but the two spent the holidays – and their birthdays – apart.
Swift, 21, rang in 2011 in Nashville, solo, at a party, while Gyllenhaal, 30, was in New York, where he attended a closing party for Broadway's Fela! on Jan. 2. While in the Big Apple, Gyllenhaal was also spotted dining with his mother at the Spotted Pig.
After spending Thanksgiving weekend together, the two were last seen publicly together on Dec. 9 in Los Angeles.
Gyllenhaal and Swift were first spotted together in October backstage at Saturday Night Live – followed by a brunch in Brooklyn.
Reporting by CARLOS GREER and KAY WEST