Justin Bieber Mourns 6-Year-Old Fan: 'Mrs. Justin Bieber' Loses Battle With Cancer

Avalanna Routh, who was known as "Mrs. Justin Bieber" and spent Valentine's Day with her favorite pop star, has died on Wednesday after a long battle against cancer.

The 6-year-old suffered from an aggressive form of brain cancer called AT/RT and was diagnosed at just 9 months old, but despite her numerous surgeries, Routh was an avid fan of Justin Bieber.

On Wednesday, Routh's parents posted the sad news on her Twitter account.

"Our darling Avalanna went to Heaven this morning," they wrote. "Oh Avalanna, the brightest star- you took our hearts with you, our greatest Love."

In February, Routh started a campaign on Facebook to fulfill her dream of meeting the 18-year-old singer, as well as her quest to pretend to "marry" him.

Bieber heard about Routh and got involved, agreeing to fulfill her wish on Valentine's Day in Boston earlier this year. On the special day, the "Boyfriend" singer posted a photo of himself and Routh with the caption, "Mrs. Bieber."

Routh's pretend marriage was reported by Boston's sports radio station WEEI. The young fans' parents, Aileen and Cameron Routh, also met Bieber as they decorated cup cakes, played board games, and signed autographs for one another.

"Hangin out with a special little girl. #MrsBieber," Bieber wrote on Twitter at the time.

Shortly after, Bieber added, "That was one of the best things I have ever done. She was AWESOME! Feeling really inspired now! #MrsBieber."

"Got some really great ideas from today," the singer also wrote on the social media site after meeting Routh. "ways to #giveback- #MrsBieber really inspired me. Really."

Bieber has responded to the sad news of Routh's death via Twitter Wednesday.

"RIP MRS.BIEBER," the singer posted along with a photo of Routh.

Additionally, dozens of Bieber fans are mourning their late fellow Bieber fan.

"Mrs. Bieber will live on in our hearts forever, #RIPAvalanna," wrote Justin Bieber Army.

Twitter user Bieber wrote, "My prayers go out to her family, her friends… & Justin. I pray they can all deal with this. Gone but never forgotten. FOREVER Mrs. Bieber."

Justin wrote, "Avalanna was a fighter! Now she's an angel in heaven! RIP Mrs. Bieber! Justin and all the Beliebers loves you!"