The couple's son, Shai Aleksander, was born this morning at 5:34 Eastern time, Chmerkovskiy announced on Twitter.
Chmerkovskiy, 36, and Murgatroyd, 30, announced their engagement in December 2015. Six months later, they revealed they were expecting their first child.
"The timing is perfect, actually," Murgatroyd told People magazine at the time. "I'll have the baby in January, and then I'll still have about six months to prepare for the wedding."
"The baby will make a wonderful addition to the wedding party," Chmerkovskiy added. "A wonderful, tiny addition."