Paulie Malignaggi suffered a unanimous decision defeat to Artem Lobov in their bare-knuckle contest on Saturday.
All of the judges scored the five-round contest 48-47 to Russian MMA fighter Lobov, despite Malignaggi coming in as the pre-fight favourite.
Former two-weight boxing world champion Malignaggi, 38, started the contest on top but claimed to have broken his hand in the second round.
By the third round, Lobov, 32, began to take the initiative and left Malignaggi reeling with a huge left hand. Lobov kept control of the fourth and final round to earn the decision win.
Both men ended the contest with bloodied faces, but Malignaggi looked to come off worse with a deep cut above his left eye.
Malignaggi confirmed his retirement from combat sport after the defeat.
"Maybe, just maybe, this wasn't meant to be," he said.
"I'm 38-years-old. I'm too old to start campaigning off of a loss. I would have done this again. I would have considered coming back. I'm done.
"I thought I was winning pretty easily. I felt I pretty much outboxed him the whole way. I'm pretty sure my right hand is broken.
"A lot of people in boxing told me you shouldn't do this and you're above this. Maybe I was wrong."