Just call her Mrs. Bieber, but no, we're not talking about Selena Gomez.

Superstar Justin Bieber spent some quality time with his new "wife" yesterday -- 6-year-old Avalanna Routh, who has a rare type of brain cancer.

Thanks to The Jimmy Fund, Avalanna was given a pretend marriage certificate, officially making her "Mrs. Bieber," and her family created a Facebook page in hopes of arranging a meeting between the two.

When a Boston affiliate of NBC News aired a story about Avalanna's lifelong battle, the young Massachusetts girl caught Bieber's attention. And just in time for Valentine's Day, he delivered.

Bieber flew Avalanna and her family to New York, where the pair spent the day playing board games, signing autographs and styling his signature hair.

"That was one of the best things I have ever done,'' Bieber later tweeted to over 17 million followers. "She was AWESOME. Feeling really inspired right now.'' The hashtag #mrsbieber was trending throughout the day yesterday as news of their Valentine's Day date spread.

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