Sarah Michelle Gellar celebrated "lifelong friend" Selma Blair's birthday on Thursday with the sweetest message.

The "Buffy the Vampire Slayer" actress, 45, took to Instagram to mark the occasion, sharing a throwback photo of herself and Blair from her own 21st birthday.

"If I told that little girl on the right, that the girl to her left was going to be her lifelong friend… she would say 'I know,'" Gellar wrote.

"From the day I met you I knew you were special and would be by my side forever," she continued, adding that she was sorry they weren't together for the special day because she has been battling COVID-19. "But I'm always with you."

Gellar and Blair first appeared together in the 1999 romantic drama "Cruel Intentions," with Gellar starring as wealthy, popular high school student Kathryn Merteuil and Blair starring as Cecile Caldwell, Kathryn's fellow student. The duo have been close friends ever since, with Gellar even telling Entertainment Weekly in 2019 that Blair had keys to her house.

Gellar ended her post on Thursday by sending love to her former co-star and calling her a "mean baby," referencing Blair's recently released book, "Mean Baby: A Memoir of Growing Up."

Blair commented on the post, writing a heartfelt reply to Gellar -- whom she called "Scarah" -- which began, "You have loved me more than any friend here."

The "Legally Blonde" actress called Gellar a "loyal and funny and supreme person," joking that her husband, Freddie Prinze Jr., is a "great guy" but doesn't hold a candle to his wife.

Blair called this birthday "majorly exciting" for her, though she admitted, "These are the moments I wish you were here. Right here. To marvel at the simple love [w]e have."

"I cannot wait to take a girls trip to celebrate all my half a century wisdom!" she added.