That's a tricky question! There are a few ways to answer it, depending on how we define "smallest":

* By diameter: Mars's moon Deimos is the smallest by diameter, measuring only 12.6 km (7.8 mi) across.

* By mass: Mars's moon Phobos is the smallest by mass, weighing in at 1.07 × 10¹⁶ kg (2.36 × 10¹⁶ lb). However, it's larger than Deimos in diameter (22.2 km or 13.8 mi).

So, depending on which factor you consider, either Deimos or Phobos would be the smallest moon.