Justin Bieber is being shady!

The "Sorry" singer is a guest on an upcoming episode of The Ellen Degeneres Show, but thanks to Snapchat we got a look backstage — and the Biebs was caught singing a Taylor Swift song!

MORE: Justin Bieber's Ex-Girlfriend Jayde Pierce is Pregnant — See Her Growing Baby Bump!

The Canadian pop star sang a verse of "I Knew You Were Trouble" on the show's social media platform, reigniting feud rumors between himself and his ex Selena Gomez's BFF.

Earlier this year, Justin, 22, was made it clear he was team Kanye West after the rapper and his wife Kim Kardashian released audio of Taylor consenting to the controversial lyrics in "Famous."

MORE: Justin Bieber, Leonardo DiCaprio, and More Stars Who Definitely Have a "Type"

"Taylor Swift what up," Justin captioned a photo of himself Facetiming with Kanye on Instagram.

Yikes! For a look back at Justin's dating history, check out the gallery below!