According to a birth certificate obtained by TMZ, the couple welcomed Amada Lee Gosling on April 29 at 8:03 a.m. at Providence St. John's Health Center in Santa Monica, California.
The newborn will join the couple's daughter Esmeralda, who will turn 2 on Sept. 12.
Mendes was spotted covering up her baby bump two weeks before giving birth. The 42-year-old actress was snapped while leaving a medical building in Los Angeles, wearing a floral maxi dress and toting a big purse.
This news comes hours before Gosling's Monday night appearance on Jimmy Kimmel Live!.
Gosling was spotted outside of Kimmel's Hollywood studio earlier in the afternoon, wearing a button-down red shirt with black jeans, sunglasses and clutching a brown bag.
Despite the couple's protectiveness over their privacy, Mendes opened to Wendy Williams about motherhood last September.
"I've never appreciated my family so much, they've been amazing," Mendes shared. "I don't technically have a nanny, but I do have help. I like to clarify. I'm certainly not anti-nanny, you do what you need to do as a parent, but I'm so grateful that I have help with my family and [Gosling's] family."