Mr Najib said the wing part found last July on France’s Reunion island was evidence that the flight tragically ended in the southern Indian Ocean.
He added that an ongoing search is expected to be completed later this year and Malaysia “remains hopeful” that the plane will be found.
If the search turns up nothing, he said Malaysia, Australia, and China will hold a meeting to determine the way forward.
The Malaysia Airlines Boeing 777 jet vanished with 239 people on board while flying from Kuala Lumpur to Beijing on March 8 2014.
“The search has been the most challenging in aviation history,” Mr Najib said in a statement. “We remain committed to doing everything within our means to solving what is an agonising mystery for the loved ones of those who were lost.”
The Australian-led search effort has spent over $130m (€118m) scouring a vast area of the Indian Ocean nearly 6.5km deep. Investigators have said the search will end by June unless fresh clues are found. Families of those on board have appealed to authorities to keep the search alive.
Transport minister Liow Tiong Lai said crews have combed three-quarters of the 120,000 sq km search zone. He said the government is waiting for verification of two more possible pieces of debris, which were discovered recently in Mozambique and Reunion island.
The investigating team issued an interim statement as required by international aviation laws on the anniversary of the plane’s disappearance, but did not provide any fresh clues about the cause.