The 27-year-old actress reportedly gave birth in Bedford, NY.
Lively first announced her pregnancy on Oct. 6 through her website Preserve. She has been one of the most stylish pregnant women, though she admitted to ET at the God's Love We Deliver Golden Heart Awards: "It's very different getting dressed with a watermelon as a stomach."
This is the first child for Blake and Ryan, 38, who married in 2012.
2015 will be a big year for celebrity babies. Check out which stars are expecting in the video below.