"You Lost Me" is a song by American recording artist Christina Aguilera from her sixth studio album Bionic (2010). It was written by Aguilera, Sia Furler and the producer Samuel Dixon. "You Lost Me" was released on June 27, 2010, by RCA Records as the third single from Bionic in the United States and the second elsewhere. The track is a down-tempo ballad that talks about an unfaithful man, who has left Aguilera's world "infected".

"You Lost Me" was well received by most critics, who noted it as one of the album's outstanding songs and praised Aguilera's vocals on the track. However, it was unsuccessful on charts worldwide. In the United States, the single reached number 20 on the Bubbling Under Hot 100 Singles, but topped the Hot Dance Club Play chart. The song had moderate success in Belgium and Israel, peaking in the top twenty in both countries.

The music video for the song was directed by Anthony Mandler. It was received positively by critics, who described it as a comeback by Aguilera following her video for Bionic's lead single "Not Myself Tonight" (2010). Aguilera performed "You Lost Me" on several television shows. The song has been covered by artists.

In August 2008, Australian recording artist Sia confirmed that she would work on several ballads for Aguilera's then-upcoming album. Aguilera commented about the collaboration, "I'm definitely a fan of Sia's. I was thrilled she also wanted to work together and, in turn, was a fan of mine". In an interview for Billboard, Sia said, "She [Aguilera] was just excited to get to work with the artists she loves. There's this misconception that she's a middle-America kind of person. But she's a little hipster. You go back to her house and sit by the fire with some wine, and what's playing over the sound system? The Knife and Arthur Russell. She doesn't listen to pop music".

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