British media report that Anthony Joshua suffered minor injuries when the car he was travelling in collided with a stationary truck on a highway between Lagos and Ogun State.
The 36-year-old Briton has relatives in the town of Sagamu in Ogun State.
Local police told the BBC that Joshua is "fine", but that two people died. Joshua has been taken to hospital.
On Monday evening, Joshua's organizers confirmed that the two deceased were the boxer's trainers and close friends.
"Our deepest condolences and prayers go out to the families and friends of all those affected, and we ask that their privacy be respected during this incredibly difficult time."
The organizers also announced that Joshua's condition is "stable", but that he will remain in the hospital for observation.
According to a witness to the accident, the BBC reports that two cars were involved: a Lexus and a Mitsubishi. Joshua was reportedly sitting behind the driver in the Lexus, which was carrying two other people.
Nigeria's Federal Road Safety Authority said in a statement that the car Joshua and the victims were traveling in was suspected of driving faster than the speed limit and losing control while overtaking.
Just over a week ago, the former heavyweight world champion defeated influencer Jake Paul in a high-profile fight in Miami. Joshua is a two-time world champion who, before turning professional, won Olympic gold in the super heavyweight division in London in 2012.
