At 310,000 sq km (119,000 sq mi) Tamu Massif is comparable in size to Mars’ vast Olympus Mons volcano – the largest in the solar system.
While Olympus Mons has relatively shallow roots, the Tamu Massif extends some 30km (18 miles) into the Earth’s crust.
The scientists say its size tops that of the previous largest volcano on earth, Mauna Loa in Hawaii.
It is located 2km below the sea on an underwater plateau known as the Shatsky Rise, about 1,600km east of Japan.
The volcano was formed about 145million years ago when massive lava flows erupted from its centre to form a broad, shield-like feature.
The researchers doubted the submerged volcano’s peak ever rose above sea level during its lifetime and say it is unlikely to erupt again.
Their findings have been presented in Nature Geoscience and one of the paper’s co-author William Sager, from the University of Houston, said Tamu Massif is relatively young.
‘The bottom line is that we think that Tamu Massif was built in a short (geologically speaking) time of one to several million years and it has been extinct since,’ he said.
The name Tamu comes from Texas A&M University, where Professor Sager previously taught before moving to the University of Houston.
He began studying the structure two decades ago, but was unsure whether the massif was one single volcano, or as is common, one of many in a single location.
Other volcanic behemoths could be lurking among the dozen or so large oceanic plateaus around the world, he said.
‘We don’t have the data to see inside them and know their structure, but it would not surprise me to find out that there are more like Tamu out there,’ said Dr Sager.
‘Indeed, the biggest oceanic plateau is Ontong Java plateau, near the equator in the Pacific, east of the Solomons Islands. It is much bigger than Tamu – it’s the size of France.’