Tori, 43, announced it in People magazine, saying:
'It was a total surprise. But we always wanted a big family. I'm really excited.'
'Dean was saying, "We just got Finn out of diapers; I thought we were in the clear!"' she added.
'For the first time this year, they're all in school. So it's like, wow. We're basically starting over.'
Tori and Dean, 49, are parents to Liam, nine, Stella, eight, Hattie, five and Finn, who's only four.
Dean is also dad to 18-year-old Jack, from his previous marriage to Canadian actress Mary Jo Eustace.
'This baby happened at the best time,' Tori said.
'Nothing is ever perfect, but I'm so madly in love with my husband and with our kids. The idea of adding to that is such a blessing.'
History in the making tonight! Watching the presidential debate with my family. It's so key that our children are part of this important historical movement. Especially raising these two #womenwarriors I love this pic @imdeanmcdermott took of me and my girls a couple years ago. We all proudly say #imwithher #presidentialdebate2016 Are you watching the presidential debate with your family?
This is very happy news, as Tori nearly lost her life during childbirth after pregnancy complications with her youngest, Finn.
Tori told US Weekly that she had been diagnosed with placenta previa at 20 weeks pregnant. Placenta previa is a condition where the placenta covers the cervix, causing internal bleeding. Tori spent 10 weeks being treated after being rushed to hospital, with four months of bed rest. She came very close to having to terminate her pregnancy to try and save her life.
Luckily, she was able to fight through it and gave birth to Finn in the August.
The couple have been through their ups-and-downs, namely when Dean was involved in a cheating scandal back in 2013. Tori spoke out about working through it, saying:
'I wanted the fairy tale! And then when it wasn't, I was like, "Well, what do I do? Divorce my husband? I have four kids,"' she told the Today show.
'I was like, "You know what? I love this man. I'm going to make this work".' We started at ground zero and built it back up. It's amazing. In May, it's going to be 10 years. Ten years! A decade.'
We wish the whole family luck and happiness in this exciting time!