The baby, whose name has yet to be announced, weighed eight pounds and nine ounces when she arrived at 4:43PM on Thursday.
Kylie shocked fans with the big news by posting an apology on Instagram for keeping the pregnancy a secret along with an 11-minute YouTube video documenting the last nine months of the new mom's life, including her physical changes and intimate moments with boyfriend Travis Scott.
"I'm sorry for keeping you in the dark through all the assumptions. I understand you're used to me bringing you along on all my journeys. My pregnancy was one I chose not to do in front of the world," Kylie captioned her post on social media.
"I knew for myself I needed to prepare for this role of a lifetime in the most positive, stress free, and healthy way I knew how. There was no gotcha moment, no big paid reveal I had planned. I knew my baby would feel every stress and every emotion so I chose to do it this way for my little life and our happiness."
Kylie continued, "Pregnancy has been the most beautiful, empowering, and life changing experience I've had in my entire life and I'm actually going to miss it. I appreciate my friends and especially my family for helping me make this special moment as private as we could."
Kylie said she "couldn't wait to share this blessing" once her baby arrived and she's "never felt love and happiness like this" before.
"I could burst!" Kylie wrote. "Thank you for understanding."
The lengthy video told a sweet story of motherhood, starting with clips of Kylie's mother Kris Jenner giving birth to her youngest daughter in the hospital. The video ended with Kylie becoming a mom herself, hearing her baby's cry for the first time and holding the little bundle of joy in her arms.
In between, Kylie shared baby-bump selfies, clips of her 20th birthday party and pink pajama-themed baby shower, her food cravings for hamburgers and cheesy French fries, moments of affection with Travis, moments of support from her sisters such as Kim Kardashian, and more.
Fans got to see Kylie's 15-week ultrasound in which Travis was by her side as well as her 25-week ultrasound with Travis' family present.
"When you're 20 years old, you're just figuring out your life," a friend of Kylie's narrates the video. "You don't know what you want. You're an indecisive teenager, you're just becoming a young adult. And there was one thing your mom knew for sure. And that was you."
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A friend can be heard saying at one point in the video with a laugh, "I knew this was what she wanted ever since she turned 15."
Another friend commented at the baby shower, "She was born to be a mom."
Kylie was four weeks apart from her sister Kim's surrogate. Kim and her husband Kanye West just welcomed a daughter, Chicago, on January 15.
Later in the video, Kris gives her future granddaughter a sweet message.
"You've got the best mom, and you're so lucky," Kris gushed. "This is such a blessing, it's going to be such an amazing journey. I can't wait to see you, meet you, kiss you, love you."
Kris and Khloe were apparently present in the delivery room when Kylie went into labor. Proud dad Travis sat by Kylie's bedside looking nervous but excited, and even Kylie's older sister Kendall Jenner FaceTimed her so she could also be a part of the delivery process.
After Kylie could be heard pushing her baby out with nothing but a black screen, the video ends with a shot of the newborn's arm and leg as Kylie looked down lovingly at her daughter, which is consistent with Kylie's desire for privacy.
Sources said her relationship with Travis became serious in June of last year, and according to TMZ, Kylie began telling her friends that she was pregnant in early September 2017.
Kylie and her ex-boyfriend, Tyga, had split in early April 2017, seemingly only days before she started hanging out with Travis, a 25-year-old rapper.