The part, known as a flaperon, was found on the shore of the French-governed island on July 29 and Malaysian authorities have said paint colour and maintenance-record matches proved it came from the missing Boeing 777 aircraft.
The French prosecutor, who had until Thursday's statement been more cautious on its provenance, said a technician from Airbus Defense and Space (ADS-SAU) in Spain, which had made the part for Boeing, had formally identified one of three numbers found on the flaperon as being the serial number of the MH370 Boeing 777.
"It is therefore possible to confirm with certainty that the flaperon found on Reunion island on July 29, 2015 corresponds to the one from flight MH370," the prosecutor said in a statement.
The plane disappeared in March last year en route from Kuala Lumpur to Beijing with 239 passengers and crew on board, most of them Chinese.
