Country newcomers Dan + Shay had an impromptu jam session at SXSW on Sunday evening (March 9), which ended, surprisingly, with a cameo in a now-viral Justin Bieber selfie!

Dan + Shay were performing for an intimate crowd of approximately 100 people at Banger’s Sausage House & Beer Garden in Austin, Texas -- an event for Scooter Braun, who manages the duo, as well as Tori Kelly and Bieber, among others.

The pair decided to participate in a jam session with Kelly, who explains, "Literally like five minutes before I was about to go on, I was like, 'We should all do a song together.'"

"So I was talking with the guys from Rixton and Cody Simpson and Dan + Shay, and I was like, 'Let's just all jam, we don't need to rehearse it,'" she adds.

As they improvised onstage, they got word that Bieber was there to surprise fans with a two-song performance. When he took the stage, UT student Maria Numez -- the selfie girl -- turned around and snapped a hilarious shot that included the superstar ... and Dan + Shay's Dan Smyers.

Now, the '19 You + Me' hitmaker is famous for more than just his catchy single.

Even more exciting than a selfie with Bieber? The news that Dan + Shay will be releasing their debut album 'Where It All Began' on April 1.