The aim of this study was to analyse the morphological and physiological maturity of the crab Aratus pisonii at Mundaú Lagoon, Alagoas, Northeastern Brazil. A total of 594 specimens were collected, being 212 males and 382 females, of which 150 were ovigerous females. More females than males were found in most months except July/2015. The morphological sexual maturity was estimated observing the morphological characters and the physiological maturity through the stages of gonadal maturation. The relationships MCPL vs. CW for males and AW vs. CW for females were chosen to represent the onset size of morphological maturity (CW50%). The CW varied from 5.13-25.11 mm for males, 6.09–27.07 mm for non ovigerous females and 11.65–23.96 mm for ovigerous females. The sizes that 50% of males reached the morphological and physiological maturity (CW50%) were 20.08 mm and 17.15 mm, respectively, and females, 17.62 mm and 13.51 mm, respectively. In this research, the physiological and morphological maturities were not synchronic for both sexes and the species matures later in the study site than in the other sites that the species was studied and this characteristic may be related to the climatic conditions and the conservation status of the mangroves.