"New Year, new family member," Brown wrote on social media. "Welcome to the family Kodi Jane. Secret (is) finally out."
Kodi weighed 7 lbs., 9 oz. and was 20.5 inches long. She joins big sister Kingsley Rose, 2.
The Browns, who were married in 2018, didn't disclose the pregnancy.
"Blessed with another beautiful angel Kodi Jane Brown… we love you so much 💕," Katelyn posted.
The new dad told People in March that having Kingsley completely changed the couple's life.
"I know when I get home and I'm exhausted, just holding her makes my day completely better," he said. "I have learned to be patient, and I've also learned that there's true love. And if anybody ever got between me and my daughter, I don't know what would go on," the musician joked at the time. "She's super chill. Kind of like me, just laid back. And if she gets mad, she's super easy to cheer up real quick."