Which volcanic mountain on Mars is the largest volcano in the Solar System?

Olympus Mons on Mars is the largest volcano in the Solar System.

Olympus Mons is a giant shield volcano built mainly by repeated eruptions of fluid basaltic lava. It rises about 22 kilometres above the surrounding plains when measured from its base, making it far taller than Mount Everest is above sea level on Earth. Its broad volcanic edifice spans hundreds of kilometres.

The volcano’s extraordinary size is partly explained by Mars’s lower gravity and the absence of Earth-like moving tectonic plates. A relatively stable crustal location could allow lava flows to accumulate in the same region for a very long time.

Olympus Mons has a large summit complex containing several overlapping calderas. No confirmed eruption has been observed, and its most recent activity is generally considered ancient on human timescales.