Justin Bieber -- Delivers Smiles for Superfan Hailey Roser Fighting Cancer

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.