The post was commemorating her 25th birthday on July 31.
"#tbt and Happy Birthday to Ms. Zelda Rae Williams!" he wrote in the caption. "Quarter of a century old today but always my baby girl. Happy Birthday @zeldawilliams Love you!"
The Crazy Ones actor was in talks to reprise his role in the sequel for the popular '90s film Mrs. Doubtfire and was working on another installment of the Night at the Museum series.
"This morning, I lost my husband and my best friend, while the world lost one of its most beloved artists and beautiful human beings. I am utterly heartbroken," his wife Susan Schneider said in a statement.
"On behalf of Robin's family, we are asking for privacy during our time of profound grief. As he is remembered, it is our hope the focus will not be on Robin's death, but on the countless moments of joy and laughter he gave to millions."
For more on Robin Williams's tragic death and his legacy of comic genius, pick up the latest issue of PEOPLE, on newsstands Friday
The Williams family is asking well-wishers to send contributions to charities close to the actor's heart in lieu of flowers. Suggested organizations include St. Jude Children's Research Hospital, Challenged Athletes, USO, the Muhammad Ali Parkinson Center, the Christopher and Dana Reeve Foundation and Glide Memorial Church in San Francisco.