"Our world is forever changed 1.10.2023," Johnson, 28, shared in a joint post with Chmerkovskiy, 36, Wednesday on Instagram, along with a black-and-white photo showing a closeup of the baby grabbing her thumb and their hands both resting in Chmerkovskiy's hand.
The couple, who tied the knot in April 2019, did not share any further details about their new addition, including his name.
Fans and friends left the pair lots of love in the comments, including fellow "DWTS" pro Cheryl Burke, who wrote, "CONGRATULATIONS!"
Chmerkovskiy's brother, Maks, left four red heart emojis on the post while his wife, Peta Murgatroyd, wrote, "He's here."
"Wowza !!!!! This is perfect! So happy for you all. A new beautiful family begins," former "DWTS" pro and current judge Derek Hough wrote.
Johnson announced her pregnancy in July, captioning a pic of Chmerkovskiy kissing her belly and writing, "Our biggest dream come true yet. Baby Chmerkovskiy coming this January! My heart has grown beyond comprehension while growing this little miracle inside of me. We can't wait to meet our precious angel."
Weeks later, Johnson revealed the sex of their baby, writing "IT'S A……BOY!!!!!!!" on Instagram alongside images of them popping blue confetti, adding, "Today we are officially half way there to meeting our son. Saying those words still makes me emotional!! SO EXCITED to be a boy mom."