The 20-year-old pop star surprised a small crowd at Banger's Sausage House & Beer Garden in Austin, Texas, on Sunday night.
Audience member Ashley Gregg said Monday that Bieber performed two songs, dedicating "Boyfriend" to ex-girlfriend Selena Gomez, who was backstage.
"This goes out to my baby girl," Bieber told a crowd of about 100 people.
The bar was hosting an event for Bieber's manager, Scooter Braun. Surprise performers included Bieber and boy band The Wanted. Cody Simpson, Dan + Shay, Tori Kelly and Rixton, who are among Braun's clients, also performed.
The music portion of South by Southwest, which will feature Lady Gaga, Coldplay, Willie Nelson and Kendrick Lamar, starts Tuesday.
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