Taylor Swift sent a sweet birthday message to her best friend, Selena Gomez, who turned 21 on Monday. She took to Twitter to wish her a happy birthday.

The “22” singer posted a picture of the two sitting on either side of mermaid statue, imitating the pose the mermaid is doing.

"Happy 21st birthday to my friend, and the closest thing I've ever had to a sister, @selenagomez!!" she wrote.

According to People, the feeling are definitely mutual, as Gomez has previously talked about her love for Swift as well.

“I met [Swift] when she was 18. This is before [Swift's 2008 album] Fearless came out, and it was absolutely incredible to see someone so successful and so humble," she said. "I think she has been a big part of me kind of staying the way I am too.”

Gomez talked to Jay Leno about how she celebrated her 21st during an appearance on Tuesday night. She said she started off the night taking a “shot of Jack.”

image: Twitter