The "Gossip Girl" alum announced her pregnancy when she accompanied Reynolds at the "Pokémon: Detective Pikachu" premiere back in May and showed off her baby bump in a body-hugging yellow Retrofête gown.
"They are so happy and excited about having another baby," a source told People at the time. "They really are the most in-love and in-tune couple, and very hands-on parents. They'd have a dozen kids if they could."
The couple, who have been married since 2012, have yet to publicly announce the baby's arrival or the gender of the child. Lively and Reynolds' new addition joins their daughters, James, 4, and Inez, 2.
Lively spoke about wanting to have a big family when she was pregnant with Inez back in 2016.
"I'm one of five kids, my husband's one of four," she explained on "Today" at the time, before joking about her future offspring. "We're officially breeders. You can go on our website and we will give you some of our children."
"She's always doing something fun and exciting," the proud mama added of daughter James. "She's the most fun, funny human being I've ever been around in my life."
The pair, who are known for being private about their kids, first debuted the adorable daughters when Reynolds got his star on the Hollywood Walk of Fame in 2016.
"I want to thank my wife, Blake, who is sitting right there who is everything to me. You are the best thing that has ever happened to me," Reynolds said during his speech at the event. "Second only to this star. You make everything better. You make absolutely everything in my life better."
The "Deadpool 2" star added, "You've given me two of the most incredible children I've ever hoped to have. You've made me the father of my dreams when I thought I only had fun uncle potential. Thank you, I love you."