Taking to her Instagram account on Tuesday, Kathryn, 32, announced they will be welcoming a sibling for their 20-month-old daughter Westlyn Reign.
"The Brolin’s are a growin'!!,” the former model captioned a snap of herself holding Westlyn Reign while showing off her baby bump. "Our little December babe is on the way…. ."
Josh, 52, quipped in the comments, "Wow. That's amazing! Congratula…wait…WHAT?!?!"
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After dating for about three years, Josh and Kathryn got hitched in North Carolina in September 2016.
They announced the birth of Westlyn Reign in November 2018 with an adorable video of the baby girl sleeping.
“Dearest Ladies and Gentlemen, I would like to introduce our little girl Westlyn Reign Brolin (Bean)," Josh captioned the Instagram post. "Mama Kathryn was stellar during this miracle birth and Bean is a flawless gem through and through. 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."
The "Avengers: Endgame" star also shares Eden Brolin, 26, and Trevor Brolin, 32, with ex-wife Alice Adair.
He was also previously married to actress Diane Lane.