The 35-year-old actress and her husband of nearly 10 years, Freddie Prinze Jr. - who have a two-year-old daughter Charlotte - are said to have been "secretly hoping" to have a son to give Charlotte a brother, so they are delighted with the news.
An insider told America`s Star magazine: "Sarah was secretly hoping for a boy. She realises this will most likely be the last baby she has, so she`s so happy she`ll have the experience of raising one of each."
Freddie, 36, is said to be "incredibly" excited that the couple - who met on the set of 1997 horror film `I Know What You Did Last Summer` and started dating three years later - are expecting a boy, and he "can`t wait" to teach his son how to play various sports.
The source added: "Freddie is incredibly pumped it`s a boy. He can`t wait to coach him in sports when he`s older."
The couple got married in Mexico in September 2002, and Sarah - whose pregnancy was announced in April - previously revealed the pair fight over who gets to stay at home and look after Charlotte.
She said: "We fight over who gets to stay home. You`d think it would be the opposite, that the parents would be like, `No, I`ll go to work!` but he`s like, `You stayed home for two years! It`s my turn!`
"He and Charlotte have too much fun. I`m jealous!"