A little girl... battling a rare form of brain cancer...is having one happy Valentine's Day.

That's because she got to fulfill her wish of meeting this guy-- pop superstar Justin Bieber!

Avalanna Routh is just six-years-old...but she's beating the odds of this aggressive, deadly cancer... that she's had since she was a baby.

She's also proving dreams... really do come true...as you can see by these pictures.

Bieber flew the little girl and her family to New York... where you can see she was all smiles.

Bieber later said it was one of the best experiences of his life and that little Avalanna was "awesome."