"Something I've been VERY excited about for a VERY long time is going to be happening VERY soon (and I'm not referring to Christmas)," she wrote, later adding: "And this is it, the big surprise... Go get it!"
She included a iTunes link to the song , a melancholy, acoustic ballad featuring the Civil Wars that includes the lyrics: "Just close your eyes / The sun is going down / You'll be all right / No one can hurt you / In the morning light / You and I'll be safe and sound."
You can listen to "Safe and Sound" here.
Not long after tweeting the link to the song, #safeandsound became the top trending topic on Twitter in the U.S. and No. 2 worldwide, with many listeners caught by surprise that Swift was part of the soundtrack. Most praised the singer and the tune itself.
Hunger Games, starring Jennifer Lawrence, Josh Hutcherson and Liam Hemsworth, hits theaters March 23. The Lionsgate movie, directed by Gary Ross, is adapted from Suzanne Collins' popular novel about a post-apocalyptic world in which teens must fight to the death in an annual event aired on live television.