Francis Ngannou said late Monday that his 15-month-old son Kobe had died.
The French-Cameroonian boxer and former MMA fighter revealed the news on social media platform X.
"Too soon to leave but yet he's gone," former UFC heavyweight champion Ngannou said.
"My little boy, my mate, my partner Kobe was full of life and joy.
"Now, he's laying without life. I shouted his name over and over but he's not responding.
"I was my best self next to him and now I have no clue of who I am. Life is so unfair to hit us where it hurts the most."
No details or cause of death have been shared.
Former two-weight champion Conor McGregor responded to the post, saying he can "only imagine the pain you are in right now".
The fighter's manager, Marquel Martin, posted: "Please respect @francis_ngannou and his family during this traumatic time. I along with millions (of) others will be praying for their strength."
Veteran ring announcer Michael Buffer said on social media: "The entire world of sports and beyond stand crushed and painfully supportive with Francis at this time.
"Please know that millions of us embrace little Kobe with our prayers."
UFC commentator and former middleweight champion, Michael Bisping also shared his heartfelt message for Ngannou saying: “I am so genuinely sorry.”
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Anthony Joshua sends Francis Ngannou to the canvas during their heavyweight bout in Riyadh, Saudi Arabia. Reuters -
Francis Ngannou lands a body shot on Anthony Joshua during their bout in Riyadh. Reuters -
Anthony Joshua's father Robert cheers on his son from ringside. Reuters -
Francis Ngannou speaks with Anthony Joshua after their bout in Riyadh. Reuters -
Anthony Joshua celebrates in the ring after winning his fight against Francis Ngannou. Reuters -
Anthony Joshua during an interview after winning the fight against Francis Ngannou. Reuters -
Anthony Joshua poses for photos after winning his fight against Francis Ngannou. Reuters -
Anthony Joshua lands the punch that knocks out Francis Ngannou. Getty Images -
Referee Ricky Gonzalez waves off the fight after Anthony Joshua knocks out Francis Ngannou. Getty Images -
Anthony Joshua throws punches at Francis Ngannou during their heavyweight bout. Getty Images
Ngannou switched from mixed martial arts to boxing last year, making his debut against WBC heavyweight champion Tyson Fury in Saudi Arabia in October.
He pushed Fury all the way, knocking down the Englishman in the third round before losing a contentious split decision.
Ngannou's next foray in the boxing ring did not go quite as well as the 37 year old suffered a second-round knockout loss to Anthony Joshua.
Englishman Joshua took to social media to express his condolences, posting a message to Ngannou saying: "May the memories you shared bring you comfort.”
A few hours before revealing Kobe's death, Ngannou had posted on X, without reference to his son: "What's the purpose of life if what we're fighting tooth and nail to get away from is what finally hit us the hardest?
"Why is life so unfair and merciless?"
