"It's a GIRL!" he tweeted. "Yippie!"
Robyn, 37, and Kody, 46, are part of a polygamous relationship, in which Kody is "spiritually married" to three other women as well -- Meri, 41, Janelle, 46, and Christine, 43.
Between Kody and his four partners, the family has a combined 17 children, and this new addition will make 18.
Kody and Robyn already share one child, 4-year-old son Solomon. She is also the mother three other children -- 15-year-old son Dayton, 13-year-old daughter Aurora and 10-year-old daughter Breanna -- from a previous marriage.
It was recently announced that Kody legally adopted Robyn's other children after they were legally wed.
The new addition to the Brown family is due on Jan. 7.