Investigators have been examining the wing part, called a flaperon, since it was flown to a French aeronautical research laboratory near Toulouse last month.
Malaysian authorities had already declared that the wing fragment was from the missing jet, but until now French investigators couldn't say with certitude that it was the case.
The Paris prosecutor's office said in a statement Thursday that investigators used maintenance records to match a serial number found on the wing part with the missing Boeing.
"Today it is possible to state with certitude that the flaperon discovered on Reunion July 29, 2015 corresponds with that of Flight MH370," the prosecutor's statement said.
The flight disappeared March 8, 2014.