"The prettiest lil thing," the "I Like It" rapper wrote.
The couple first announced they were expecting in June.
The singer/actress shared her joy, but also disappointment that tabloid photographers disrespected her family's privacy.
"Papa Bear dropped a surprise on this pod today," Bunnie XO writes of the country star's revelation on Monday's Bussin' With the Boys podcast.
"Best day of my life," the model and social media personality captioned her announcement.
"Pure moments of magic," the star wrote.
Waterhouse revealed that she was pregnant back in November.
The couple also share two girls, Gio Grace, 5, and Dusty Rose, 7.
The parents-to-be threw a lavish Disney-themed baby shower over the weekend. Get all the details.
The singer said the couple's second child was born on Saturday.
