Khloe Kardashian is a mother.
Thompson was excused from practice for the birth, the Cavs added.
TMZ was the first to report the news, stating that the baby was born on Thursday morning in a hospital outside Cleveland.
Representatives for the NBA team and Kardashian have not responded to ABC News' requests for comment.
This is the first child for Kardashian; Thompson has a 1-year-old son, Prince, from a previous relationship.
Kardashian, 33, and Thompson, 27, were first linked in the 2016 and announced in December 2017 that they were expecting a baby. Several weeks later, the "Keeping Up With the Kardashians" star revealed on her reality show that she was pregnant with a girl.
The birth comes on the heels of reports that Thompson cheated on Kardashian during her pregnancy. The Daily Mail published a photo that seems to show Thompson kissing a woman last weekend; video footage from TMZ appears to show the NBA star kissing a woman inside a nightclub last October.
Neither Thompson nor Kardashian have commented on the reports.
This is the third new addition to the Kardashian/Jenner clan this year. Kim Kardashian West and Kanye West welcomed their daughter, Chicago, in January, and in February, Kylie Jenner gave birth to Stormi, her daughter with rapper Travis Scott.