The 41-year-old actress marked the occasion Tuesday by sharing a funny adage about marriage and a photo with the 42-year-old actor.
The picture shows Gellar and Prinze smiling for a selfie in a parking lot. The couple officially celebrated their anniversary Sept. 1.
"Anniversary #latergram," Gellar wrote. "Someone told me - Marriage is like a deck of cards. In the beginning all you need is two hearts and a diamond. But years later, you wish you had a club and a spade. #happyanniversary @realfreddieprinze."
Gellar had also posted a playful message on Prinze's birthday in March.
"Gorgeous, friendly, sexy, sweet, intelligent, super talented, kind, charming, funny, witty, amazing smile.... well that's enough about me.... #happybirthday @realfreddieprinze (you're pretty great too)," she wrote.
Gellar and Prinze shared a funny moment in July when Prinze pushed Gellar's face into a cake to help her celebrate two million Instagram followers. Prinze was more serious when he discussed the secrets behind his marriage with Us Weekly in September 2017.
"We treat each other with respect. We didn't just get married because we thought the other was young and hot," the actor said.
"Sacrifice is something that's required in a marriage. It's fun being single, right? You can be selfish," he added. "But marriages require a step back from that and a bit of sacrifice, and our sacrifice I think is maybe the biggest thing that's made our marriage work."
Gellar and Prinze married in 2002 after meeting on the set of the 1997 movie I Know What You Did Last Summer. The couple have two children, 8-year-old daughter Charlotte and 6-year-old son Rocky.