The couple have named their first child Westlyn Reign Brolin, with Bean being her nickname.
The "Avengers: Infinity War" star shared the news on Instagram, Sunday.
"We are blessed to the core and appreciate so much the love and support during this pregnancy from all who have partaken in our journey, directly and/or indirectly," Brolin, 50, wrote alongside the little one's video.
An overwhelmed Boyd, 31, shared the same video on Instagram.
"Our family's newest (and tiniest) member. Westlyn Reign Brolin, I love you so much already. Nothing compares to this... Nothing. My heart is totally different forever. @joshbrolin thanks for this little life of ours," the model wrote.
The couple, who tied the knot in 2016, announced their pregnancy in June.
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