When the big-hearted pop pin up heard that it was Avalanna Routh's dream to meet him, he flew her and her family from Boston to New York.
Now six, the little girl has been fighting from brain cancer since she was born and has had several rounds of chemotherapy and radiation.
After spending the afternoon playing board games, the pair decided to get 'married', eating cupcakes, signing autographs for each other before.
Avalanna even styled her 17-year-old hero's hair.
Asked how long she'd loved him, the devoted fan replied: "Seven years. No - 80 years."
The teenager tweeted a picture of them together, telling followers: "That was one of the best things i have ever done. she was AWESOME! Feeling really inspired now!', before adding: '#MrsBieber really inspired me."
His real girlfriend, Selena Gomez, 19, would understand why he wasn't able to spend the day with her.
In any case, Justin let her know that she was in his thoughts by giving her a diamond promise ring – a kind of pre-engagement ring – bearing his initial.