The huge Tamu Massif was found 1600 miles east of Japan and is comparable in size to the extinct Olympus Mons volcano on Mars, which is the largest in the Solar System.
The new discovery, which lies more than a mile below the sea, topples the previous largest on Earth, Mauna Loa in Hawaii.
A team writing in the journal Nature Geoscience says the 120,000 square mile volcano is located on a plateau known as the Shatsky Rise.
It was formed about 145 million years ago when massive lava flows erupted from the its centre to form a broad, shield-like feature.
The researchers doubt the submerged volcano's peak ever rose above sea level during its lifetime and say it is unlikely to erupt again.
"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," said co-author William Sager, from the University of Houston in Texas.
"One interesting angle is that there were lots of oceanic plateaus (that) erupted during the Cretaceous Period (145-65 million years ago) but we don't see them since. Scientists would like to know why."
Professor Sager began studying the structure two decades ago, but it had been unclear whether the massif was one single volcano or many.