This will be Gellar and Prinze Jr's second child together. The couple, who have been married for ten years, welcomed daughter Charlotte Grace in September 2009.
Gellar said of motherhood following Charlotte's birth: "You need to be at a point in your life where you can give up anything and everything for a child."
Sources close to the expecting parents told People that the couple are thrilled about the news.
"They love their little girl more than anything in the world and know that love will only multiply. She is such a fantastic mum; it is just great news," one insider was quoted as saying.
> Sarah Michelle Gellar: 'Daughter thinks job is getting my hair done'
Gellar, who rose to fame on the '90s television series Buffy the Vampire Slayer, stars in The CW drama Ringer.
The show is currently waiting to hear if it has been picked up for a second season.
Prinze Jr has signed on to star in an untitled NBC comedy pilot opposite Mena Suvari, from the writers of Community.
Watch a clip of Sarah Michelle Gellar on Ringer below: