The debris is apparently the first piece of physical evidence recovered from the ill-fated plane, which disappeared on March 8, 2014 along with its 239 passengers and crew.
“It is with a very heavy heart that I must tell you, an international team of experts have conclusively confirmed that the aircraft debris found on Reunion is indeed from MH370,” Prime Minister Najib Razak said at a brief press conference. "We now have physical evidence that ... Flight MH370 tragically ended in the Southern Indian Ocean."
Meanwhile, French authorities said only that there were "strong presumptions" the part was from MH370.
An environmental worker discovered the debris -– identified as a “flaperon,” a Boeing 777 wing segment -– on a beach on Reunion Island last week. The barnacle-crusted flaperon was later transported to Toulouse, France for further investigation.
Investigators are hoping the part can provide some clues about the jet’s final moments.
Some experts have postulated that the damage suggests the flaperon may have been deployed when the plane hit the water, meaning that someone in the cockpit was consciously manipulating the controls.
“We’ll have to look at that piece and have to gather every bit of information from it that we can,” said Former NTSB Office of Aviation Safety Director Tom Haueter, an ABC News contributor. "We might get a better idea of what happened to this airplane during the final moments of flight, when it hit the ocean."
Meanwhile, the search continues for the submerged wreckage, and people are combing beaches for more floating debris.
So far, however, none of the items turned over to the authorities -– including a mangled piece of metal that officials say turned out to be part of a domestic ladder -– appear to come from MH370, according to the Australian Transport Safety Bureau.
"The burden and uncertainty faced by the families during this time has been unspeakable," Prime Minister Najib said today. "I promise you this,Malaysia will always remember and honor those who were lost onboard MH370."
In a statement, Malaysia Airlines confirmed that the families of the passengers and crew, who are all presumed deceased, had been informed about the part's linkage to the missing craft.
"We extend our deepest sympathies to those affected," the airline said. "We expect and hope that there would be more objects to be found which would be able to help resolve this mystery."
Get real-time updates as this story unfolds. To start, just "star" this story in ABC News' phone app. Download ABC News for iPhone here or ABC News for Android here. To be notified about our live weekend digital reports, tap here.
ABC News' Matt Hosford, Hugo Leenhardt, Josh Hoyos, Kirit Radia, and Joseph Simonetti contributed to this report.