"Happy anniversary @realfreddieprinze," she posted along with an old photo of the two. "You stole my heart, so I got my revenge and took your last name!!"
The couple met on the set of the 1997 horror film I Know What You Did Last Summer and married in Mexico in 2002. They are the parents of 6-year-old Charlotte Grace and 3-year-old Rocky James.
In an interview with People earlier this year, Gellar shared some of her best relationship advice.
"Make the most of the moments you have," she said. "Take the 10 minutes – put the phone down. Have a cup of coffee together. Walk the dog at the end of the night. Read a story with your kids."