What is the name of Mars’s largest moon by diameter?

Phobos is Mars’s largest moon by diameter.

Phobos measures about 27 by 22 by 18 kilometres, making it much larger than Mars’s other moon, Deimos. It orbits extremely close to Mars, completing one orbit in about 7 hours 39 minutes—faster than Mars rotates.

Because Phobos travels around Mars faster than Mars turns, it rises in the west and sets in the east as seen from much of the Martian surface. Its heavily cratered, irregular body is probably an asteroid-like object, although its exact origin remains debated.

Phobos is gradually spiraling inward because of tidal interactions with Mars. In roughly tens of millions of years, it may break apart or collide with the planet. Phobos is not the largest moon in the Solar System; that title belongs to Jupiter’s Ganymede.