"My husband thinks I'm crazy, but I'm not the one who married me. #happyanniversary @realfreddieprinze 15 years!!" Gellar wrote alongside a slideshow of images from their relationship through the years.
Gellar, 40, and Prinze Jr., 41, co-starred in the movies I Know What You Did Last Summer, I Still Know What You Did Last Summer, Scooby-Doo and Scooby-Doo 2: Monsters Unleashed.
They have two children -- daughter Charlotte, 8, and son Rocky, 5.