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On Monday, the NBA player shared a message on his Instagram Story, writing: "Today, paternity test results reveal that I fathered a child with Maralee Nichols. I take full responsibility for my actions. Now that paternity has been established I look forward to amicably raising our son."
"I sincerely apologize to everyone I've hurt or disappointed throughout this ordeal both publicly and privately," he continued.
Thompson, 30, had previously disputed the paternity of the child, who was born on Dec. 1.
"Khloé, you don't deserve this. You don't deserve the heartache and humiliation I have caused you," he wrote.
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"You don't deserve the way I have treated you over the years. My actions certainly have not lined up with the way I view you. I have the utmost respect and love for you. Regardless of what you may think. Again, I am so incredibly sorry."
Thompson previously denied having sex with Nichols in California where the paternity case was filed, claiming that the "only" possible date of conception for the child is March 13, 2021, which is his birthday when he was in Texas. In previous court documents and in a statement last month, Nichols said the child was conceived in April.
A source previously told PEOPLE that the Keeping Up with the Kardashians alum, 37, and Thompson officially split in June. The exes share a 3-year-old daughter True. (Thompson is also dad to son Prince, 5, whose mother is ex Jordan Craig.)
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In court documents, Thompson claimed he only communicated with Nichols via Snapchat, the photo messaging app that automatically deletes messages after they're read, and repeatedly asserted that his relationship with Nichols was "based on sex only."
"We did not have any dates in the traditional sense at any time; there were no dining at restaurants, going to movies, traveling or any other indicia of a normalized relationship," he said. "There was only Snapchats of 'where' and 'what time' we would hook up and what hotels would be used."
In a lengthy statement issued last month, Nichols disputed Thompson's claim that their past was "based on sex only."
"Tristan told me he was not having a sexual relationship with anyone else; it was not casual sex nor did we have any arrangement. Tristan and I communicated daily and talked on the phone. We would see each other several times a month," Nichols said, denying that they communicated solely through Snapchat.
Nichols' paternity suit against Thompson, which was filed in Los Angeles, is still pending. She is suing the NBA athlete for pregnancy-related fees and child support. A parallel paternity suit filed by the Sacramento Kings player against Nichols was dismissed by a Texas judge in December.