"Laying in bed watching my friend @sarahmgellar on #watchwhathappenslive," Doherty, 45, wrote. "Sarah you are a class act. From your pinky toe to the top of your head. I am so very proud of you and not just for your success with @foodstirs or your amazing career or your new book, #stirringupfunwithfood but for the warm, generous, supportive, loving person you are."
"I'm privileged to know you and love you. #proudofmygirl," she added.
ET spoke with 39-year-old Gellar on Monday in New York City at the Good Housekeeping Test Kitchen in New York City, where she opened up about their friendship, and Doherty's openness about her cancer battle.
"Shannen's incredible," Gellar gushed. "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. And it's OK to be vulnerable, and you can do it."
The Buffy the Vampire Slayer star also said Doherty inspires her.
“The fact that she gets up and goes to those dance classes? The last thing I want to do is go to these really hard dance classes, and sweat that hard and work that hard,” Gellar admitted, “but I have to remind myself, if she's getting up there and doing it, then it's important that I do it, too."