The former couple announced they were expecting a sibling for daughter True in July.
"Khloe Kardashian and Tristan Thompson welcomed their second child, a baby boy, via surrogacy," the source says. A rep for The Kardashians star also confirmed the news to Page Six.
Khloe, 38, and Tristan, 31, announced that they were expecting their second child together in mid-July, however, a source told ET at the time that the former couple was not getting back together, following their split over Tristan's paternity scandal.
"The two aren’t together and Khloe will have the baby full time," the source shared, adding that "Khloe wants Tristan in both of the kids' lives as much as he wants to be."
Another source added, "Khloe and Tristan are not back together and have not spoken since December outside of co-parenting matters."
The decision to have another child together came before Khloe learned that Tristan was expecting another child with Maralee Nichols in December, a rep for the Good American founder confirmed to ET at the time.
"We can confirm True will have a sibling who was conceived in November," the rep said. "Khloe is incredibly grateful to the extraordinary surrogate for such a beautiful blessing. We’d like to ask for kindness and privacy so that Khloe can focus on her family."
Meanwhile, one of ET's sources says that Khloe and Tristan "have long said they wanted True to have a sibling, and Khloe even told her family she’d do it without Tristan if she needed to."
Khloe eventually decided against doing it sans Tristan. As she and the pro athlete "were on good terms, they decided to move forward with surrogacy near the end of last year," the source said, adding that it was only after that decision was made that "Tristan was caught cheating."
"I feel incredibly let down," Khloe said in the season 1 finale. "There’s nothing that can be said or done that’ll make me feel better. All of the work, the energy, the conversations, all of those bricks that we were laying down [on] this foundation all for a tornado to go through and everything just be demolished."
"Sometimes you do stupid things for your kids, but also you can’t be stupid for your kids for so long," she added. "I will say I’m really proud for how I tried for True and the rest is the rest."
See more on the former couple in the video below.