A team writing in the journal Nature Geoscience says the 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.
The structure topples the previous largest on Earth, Mauna Loa in Hawaii.
The massif lies some 2km below the sea.
It is located on an underwater plateau known as the Shatsky Rise, about 1,600km east of Japan.
It was formed about 145 million years ago when massive lava flows erupted from the centre of the volcano to form a broad, shield-like feature.