Sandy beaches are dynamic and challenging ecosystems. The structure of the resident macrofaunal communities primarily arises from independent responses of individual species to physical environmental variables. For supralittoral species, the Habitat Safety Hypothesis (HSH) predicts an increase in population attributes from dissipative beaches (fine sand, gentle slope) to reflective beaches (coarse sand, steep slope). Salinity is considered an additional environmental stressor, particularly in transitional environments characterized by strong gradients. The simultaneous effects of morphodynamics and salinity on arachnids have not been tested before. We examined the effect of morphodynamics and salinity on the abundance and body traits of the sand-dwelling spider Allocosa senex. We conducted concurrent samplings of adult individuals in three pairs of beaches with contrasting morphodynamics, representing three distinct salinity regimes along the Uruguayan coast. Our findings aligned with the predictions of the HSH. Males were more abundant on reflective beaches, and in those beaches with lower salinities. Both sexes exhibited larger size, weight, and body condition in reflective beaches and also a decrease from inner-estuarine to oceanic beaches. Spiderling sizes also followed the same trend. These results emphasize the importance of considering both morphodynamics and salinity as critical factors shaping population attributes in sandy-beach arthropods.