Kutcher wrote about the birth on his website, Aplus.com.
"Mila and I would like to welcome Wyatt Isabelle Kutcher to the world. May your life be filled with wonder, love, laughter, health, happiness, curiosity, and privacy," he wrote.
Underneath the message, he posted several baby photos, saying one of them is of their child.
The pair, engaged early in 2014, initially kept quiet about the pregnancy. Kunis, 31, addressed the rumors in May and confirmed she was expecting.
She opened up to Ellen DeGeneres on her show and spilled all the details of her pregnancy, including her plans to have a natural birth. The "That 70's Show" co-stars even considered having the baby at home, but decided that wasn't the best option for them.
"We watched a couple of documentaries and we looked into the like the midwife aspect of it and things and spoke to my OB/GYN and realized that the hospital that I'm going to be laboring in does a midwife, you know, doula type of thing, and so I'm gonna do it as all natural as I possibly can unless that there's an emergency or something that should wrong," Kunis told DeGeneres.
The "Jobs" star, 36, and his pregnant fiancee weren't just quiet about the pregnancy, but the engagement as well. Kunis said she was wearing an engagement ring for almost two months before anyone noticed in February.
"I think everybody assumed we've been engaged so long that I was wearing the ring in public for two months and no photograph was ever...like nobody ever commented on a single photograph. Like it was just not...they just decided not to acknowledge it for two months. It was amazing."
Kunis told Marie Claire that when it comes to her baby with Kutcher, she just wants a nice kid.
"I just don't want my kids to be a**holes. I feel like so many kids nowadays are," she told the magazine. "Both of us just want to have well-behaved, honest, nice children who people meet and say, 'That's a good kid.'""
Kunis and Kutcher were friends and knew each other for almost two decades before they two got together in 2012. She even said Kutcher was her first real kiss, the two smooching as kids on "That 70's Show."
Since then, Kunis has gone on to star in movies like "Ten," while Kutcher has landed big roles, included on the hit show "Two and a Half Men."
Kutcher was previously married to actress Demi Moore for seven years, until their separation in 2011. Their divorce was finalized in November of last year.