Taylor Swift to Selena Gomez: You are the 'closest thing I've ever had to a ...

taylorswift13/via Twitter

Looks like Taylor Swift and Selena Gomez will never ever not be best buds.

Swift took to Twitter Monday to post a sweet message to Gomez on her 21st birthday expressing that the two Hollywood starlets have a sisterly bond.

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"Happy 21st birthday to my friend, and the closest thing I've ever had to a sister, @selenagomez!!" Swift posted along with an adorable picture of the BFFs.

The gals can be seen perched up on a balcony above a beach mimicking the pose of a mermaid statue placed in between them.

RELATED: WATCH: SELENA GOMEZ DEBUTS 'BIRTHDAY' MUSIC VIDEO

Aside from Gomez's A-list birthday wishes, the now 21-year-old brunette beauty has had a huge week for her music career.

The "Come & Get It" singer's new album Stars Dance was released Tuesday and quickly climbed to number one spot on the iTunes chart.

RELATED: SELENA GOMEZ'S 'STARS DANCE': MUSIC REVIEW

"#1 AHHHHHHHHHHHHH!!!!!" Gomez tweeted to her over 15 million followers.

"I am still in shock, I cannot even believe it right now. I love you guys so much, THANK YOU!!! Now let's party!!!" she wrote on her Facebook page to also address her many fans.