Allocosa senex is distributed along the sandy coasts of Argentina, Brazil and Uruguay. Individuals are nocturnal and build burrows in the sand, where they stay during the day and in the Southern Hemisphere winter. Most studies of this species have been carried out in estuarine or oceanic beaches. A. senex faces an important environmental stress because its habitat, particularly in coastal lagoons, is affected by periodic floods. The behavioural responses of individuals of this species under those conditions are unknown. Our objective was to estimate the abundance, density, and cursorial dispersal of A. senex in two beaches of a coastal lagoon in Rocha, Uruguay. Additionally, we tested A. senex behavioural responses to floods under laboratory conditions. The results showed that the abundance and dispersal ability of A. senex individuals are higher at sites with greater availability of suitable habitat. Females were the most mobile sex and showed the highest recapture frequencies. A. senex individuals survived artificial floods and displayed swimming and diving behaviours. These results suggest a high degree of adaptation of this wolf spider to inhabit coastal environments and cope with its extreme variations.