Gellar took to Instagram to honor her "most loyal friend," alongside a cute throwback pic of the two.
"Where do I even start with this birthday girl?!? She's everything," the 39-year-old actress wrote. "She's inspired me...well since... even before I called her a friend. But over the last year, I have watched her inspire and give hope to the world."
"I thought I knew how tough she was, but I didn't even have a clue to her power. When I think twice about leaving the house without makeup, Shannen shaved her head on the public stage. While I fiercely guard my privacy, she opened her life, her struggles, her fears and her journey to help people she has never even met," she continued. "She is a true friend to everyone."
"Shannen also has the greatest laugh in the world, which is why I chose this picture. To the fiercest, most loyal friend I know....let's party like it's your birthday!!! I love you," Gellar concluded her touching message.
ET caught up with Gellar last week in New York City, where she gushed about her "incredible" friend.
"Shannen's incredible. What she's done for other women going through it is... so brave, and that's what real bravery is, is to go through the hardest part of your life and show people, it's OK to be scared," Gellar said of Doherty's willingness to share her breast cancer battle. "And it's OK to be vulnerable, and you can do it."
See more on the pair's friendship in the video below.