Silviu Ionescu, Romania’s charge d’affaires in Singapore at the time, had been sentenced to six years behind bars in February by a Bucharest appeals court. He died of cardio-respiratory problems, judiciary sources told Mediafax news agency.
Ionescu was found guilty of manslaughter for hitting three pedestrians in December 2009 while driving a car belonging to the Romanian mission.
One of the pedestrians, a 30-year-old Malaysian, died.
The court also found him guilty of inflicting injuries and fleeing from the location of the crash.
During his trial, which started in 2010, the ex-diplomat repeatedly said he was framed by the Singapore authorities because of critical reports he sent to the Romanian foreign ministry as head of the diplomatic mission in the city-state.
His defence team argued during the trial that he should be acquitted on the grounds that there was no direct evidence linking him to the crime.
Ionescu fled Singapore shortly after the accident.