The Victoria`s Secret model and her husband Griffin Guess welcomed son Gavin Lee Guess in Santa Cruz, California on 13.12.12.
The tot weighed 8 lbs., 10 oz. and measured 22 inches long.
Marisa told People magazine: "I-m completely overjoyed by the birth of our son. I feel so blessed to be a mom and am so excited to experience this next part of life."
The couple revealed Marisa was expecting in June and although the model complained of being "exhausted" and "gaining weight", she was excited about becoming a mother.
She said: "My husband and I have always looked forward to starting a family, but also wanted to enjoy just being married. After six years together, the time feels right. My sisters and I will all have babies in 2012 so it`s such an exciting time. Even though I`m exhausted, gaining weight and breaking out from all the hormones, I`m really enjoying my pregnancy. It`s all for the baby so I couldn`t care less!"
The couple tied the knot in April 2006 and Marisa has previously revealed she only has eyes for her husband.
She said: "My job can definitely bring out a lot of insecurities or trust issues, but I`m so thankful we never had that. I don`t give my husband a reason to worry. Those kind of things can become a big problem in any relationship, and then especially with [my life] there`s a whole new set of challenges."