Johngarthia malpilensis (Faxon, 1893) is the least studied of the eight American species of Gecarcinidae. This land crab is considered endemic to Malpelo, an oceanic island of the Colombian Pacific. Several aspects of its ecology were investigated between 2003 and 2006. We estimated its population density, distribution, daily activity, reproduction, interactions, and diet by marking and monitoring 909 individuals. During our visits we recorded crabs of sizes from 5 to 82 mm carapace width. Johngarthia malpilensis shelters mainly in fissures and hollows between rocks. It is distributed all over the main island except in very steep sectors. An average density of 0.41 adults m−2 and 0.55 juveniles m−2 produced an estimated total population of 833,000. Johngarthia malpilensis showed high mobility, with crabs covering distances over 450 m in a few days on highly irregular surfaces. Activity was higher from dusk till dawn and lowest around noon. Release of larvae took place during the high tides associated with the new moon, at least during the rainy season. It is omnivorous and opportunistic, consuming practically every available resource. The crab is occasionally preyed upon by an endemic lizard and migratory birds. Its general ecology is very similar to that of J. planatus, a closely related species. As a voracious omnivore J. malpilensis is one of the most important components of Malpelo's food web.