"The most beautiful surprise on this very special day," she captioned a picture of herself and hubby Samuel Lightner Cusick in the hospital with their new arrival.
"Baby Girl and mama are healthy and well," she continued with some heart emojis. "More details to come but wanted to share this first pic of our little family."
Arnold's DTWS costars took to the comments section to congratulate the new mom on the exciting milestone.
"My heart," Witney Carson wrote. "She’s so perfect in every way! SO SO HAPPY FOR YOU TWO! I can’t wait to snuggle her!!!!!!"
"Absolutely stunning!!!!!! I love you guys," Alan Bertson said.
"What a BEAUTY!" Peta Murgatroyd commented.
Arnold could not compete on this season of DWTS as she was pregnant. A few months later, fellow DTWS pro and bestie Carson announced that she was pregnant, too.
"Ohhhhhh baby," Arnold began her pregnancy announcement in May. "Mom and Dad love you already."
Since then, the dancer has been regularly updating her followers on her pregnancy journey.
"Being confident and loving yourself is one of the best things my mom ever taught me, and it’s what I will teach Baby Girl one day. This isn’t always easy, and it’s an ongoing journey, but once you realize YOU are good enough, the rest will follow!" she wrote on Saturday, October 3.
"Not going to lie, it’s been difficult seeing my skin change so much during pregnancy, but that’s not what’s important! I’m growing a human inside of me, of course my body and skin are going to change!" she added. "I challenge you all to tell yourself you are good enough and to stop comparing yourself to others! We're all in this together, so let's all lift each other up and be kind to one another!"
Although she wasn’t dancing on this season of the show, she was still very active throughout her pregnancy. "38 weeks pregnant and still moving my body!" she shared last week. "This was a goal of mine and I’m so grateful and amazed that I’m still able to get up and move!"
For Halloween, the couple had the genius idea of dressing her baby bump as a basketball, while her husband wore a Lakers jersey.
She first joined DWTS in season 16 in 2013 and was the second youngest pro-dancer in the show’s history at the time.
The dancer married Cusick in 2015, who was her high school sweetheart.
Congrats to the happy couple!