The amounts of leaf litterfall and leaf litter removal by the feeding of a numerically abundant sesarmid crab Parasesarma bidens were examined in a subtropical mangrove forest, dominated by the red mangrove Rhizophora stylosa, on Iriomote Island, southern Japan, in 2016. Field observations and stable isotope analyses showed that P. bidens consumed and assimilated leaf litter rather than other primary food sources such as macroalgae and microphytobenthos. Measurements of the leaf litterfall amount were conducted using litter traps for four weeks in May and June. The weekly and estimated annual litterfall amounts were 3.5 g/0.25 m² and 730g/m² （dry weight), respectively. Leaf litter consumption by P. bidens, determined using a field cage experiment in August, indicated that a single crab consumed 1.5 g/week（dry weight), the estimated annual consumption under natural crab density（8 individuals/m²）being 626 g/m² （86% of total litterfall amount). We conclude that leaf litter feeding by P. bidens significantly contributes to the organic matter cycling within the mangrove ecosystem.