Justin Bieber has visited the bedside of a fan battling leukemia.

The 'Baby' hitmaker surprised seven-year-old Millie Flamm in her hospital room at the Primary Children's Medical Center in Salt Lake City, Utah, on Saturday evening when he heard she was too ill to attend his concert later that night.

Friends and family of Millie launched an online campaign urging the 18-year-old pop star to visit after she was forced to sell her tickets two weeks prior to the gig due to her suffering from a relapse in her cancer fight.

Her mother, Amanda Flamm, revealed her daughter "lit up" when Justin walked into the room and hugged her.

He then performed his song 'Baby' for her and gave her his guitar pick as a memento from his visit.

Amanda told local newspaper the Deseret News: "All of a sudden it pulled through and we were able to do a really intimate little meeting with just her and us and Justin and it was incredible and he was so sweet to her.

"He walked in and her whole face just lit up. She is squeezing on to it (the guitar pick) with all her might and will not let it go."

Justin posted a photo of himself and Millie cuddling on his Instagram account, adding the caption: "Look at this bundle of joy... I love her."

After he gave her a kiss on the cheek and left, Millie gushed to her mother: "You are never going to wash my face again!"