The 38-year-old actress, who played Williams' daughter on sitcom "The Crazy Ones", took to Instagram on July 21 to honour him with an adorable snap of the two together and a sweet birthday message, reported People magazine.
The picture showed Gellar and Williams hugging each other just like a father and his daughter.
They were smiling ear to ear in the black-and-white picture. A caption for the picture read, "Smile, though your heart is aching. Smile, even though it's breaking. Happy Birthday RW #RobinWilliams #HappyBirthdayRobin."
On the previous day, Williams' daughter Zelda announced her hiatus from social media with a picture of big cloud with halo on Instagram.