This paper describes seasonal changes (summer vs. winter) in reproductive traits (fecundity in relation to female body size and dry mass, egg size, egg weight and reproductive output) of the semiterrestrial crab Sesarma rectum from a mangrove area (23°29′S, 45°09′W) on the northern coast of the State of São Paulo, Brazil. Regression analyses using log-transformed data were used to test size-dependent relationships, which were compared between the breeding periods by means of ANCOVA. During winter, the eggs produced were heavier and larger; therefore, no trade-off between egg number and female size was observed. The mean size of ovigerous females remained fairly constant across the seasons. In contrast, the reproductive output and mean dry weight of females decreased in summer. These trends may indicate the seasonal variation of energy allocate for reproduction. The ecological implications of the observed seasonal trends are discussed in the context of the temporal variation of environmental and biological factors in the region.