Mission accomplished for one of Justin Bieber's biggest fans -- a little girl who is fighting cancer, got a private meeting with JB after a huge social media campaign that included a call to TMZ Live.
4-year-old Hailey Roser was all smiles during the 5 minute pow wow backstage with the Biebs before his concert in Milwaukee Sunday night.
Hailey's parents started the "Healing Hailey" campaign on Twitter and Facebook after she was diagnosed with a brain tumor in August -- and a few weeks ago one of the campaign's followers called in to TMZ Live to spread the word.
We joked with the caller, saying ... "we know some people" -- and then informed Justin's manager Scooter Braun about the "Healing Hailey" movement.
Hailey's been undergoing chemotherapy and had to get clearance from doctor's today to attend the concert. After meeting Justin, she tweeted a message to her followers ... "Dreams come true #believe"