The 41-year-old actress already has a five-year-old daughter, Harper. Shetook to Instagram to share the first snap of her two children together.
Captioning the photo, Thiessen wrote: "Yesterday Harper got a little brother!
"We are so excited to introduce Holt Fisher Smith. Our perfect sweet little man. #weareblessed #familyoffour"
Harper looks proud to be holding her sleeping brother, as she had been excitedly waiting for this moment for a long time.
Thiessen told online magazine Sweden With Love last year: "Every day she [Harper] begs me to have a baby.
"Every morning she wakes up and says, ‘Is there a baby in your belly?’ She thinks it grows overnight while we’re sleeping. Then she looks at me and says, ‘You’re not growing a baby, Mommy!'"
The actress told People magazine when she announced her pregnancy: "Harper is beyond thrilled to be having a playmate!"
Thiessen married her actor husband in July 2005 and they welcomed their first child five years later.
On Harper's fifth birthday Thiessen shared a throwback photo of herself with her daughter, when she was a baby.
She wrote: "Thank you for allowing me to be your mommy. I love you."