The singer, 19, collapsed on stage at London's O2 arena on Thursday night after complaining of breathing problems throughout the performance.
Early Friday morning London time, he Tweeted: "getting better. thanks for everyone pulling me thru tonight. best fans in the world. figuring out what happened. thanks for the love"
"He got oxygen from emergency personnel. He insisted on finishing the show," his rep tells E!. "He is on the way to the hospital right now."
Bieber's manager, Scooter Braun, got on stage to explain to the crowd what was going on, according to YouTube videos of the show.
"Justin got very light of breath. The whole show he's been complaining, and he's backstage with the EMTs and the doctor," Braun announced. "They're telling him that they want him to go see somebody and see what's going on with his lungs."
Bieber then returned for a rousing ovation, continued the performance, before being taken to the hospital.
This is the latest problem to plague Bieber's U.K. trip.
He earlier apologized for going on late for a show, complained about his "worst birthday ever" at a local club, was mocked by Olivia Wilde, and finally went on a Twitter rant.