The 18-year-old 'Baby' hitmaker heard about the plight of four-year-old Hailey Roser after her parents started an online campaign called Healing Hailey following her brain tumour diagnosis in August, reported Sun online.
Bieber decided to make the youngster's day by inviting her to meet him for a chat backstage at his sold-out show at the Bradley Center arena in Milwaukee.
Roser, who has been undergoing chemotherapy, was given clearance from her doctor to attend the gig, and she posed for pictures with Bieber shortly before he took the stage.
A message on Roser's fund raising Twitter page reads, "I am so exited for Hailey right now, she met Justin Bieber!!! She deserved every second of it! Thank you."