The aim of this study was to investigate the population biology of the green porcelain crab Petrolisthes armatus in an estuarine area of northeastern Brazil. Individuals were monthly sampled (February 2011-January 2012) during the spring low tide. Four transects, 10 m apart from each other and perpendicular to the shoreline were delimited in the rocky zone of a beach, comprising an area of approximately 861 m². Crab sampling was carried out using 0.25-m² plots (10 equally spaced plots were sampled from each transect per month). A total of 836 individuals were collected: 401 males (48%), 220 non-ovigerous females (26.3%), and 215 ovigerous females (25.7%). Ovigerous females were significantly larger than males and non-ovigerous females, while males were significantly larger than non-ovigerous females. The overall sex ratio of 1:0.9 did not differ significantly from the expected 1:1. The highest abundance (19.8 ind.m-2) was observed in transect 3, in the mid-intertidal, suggesting zonation and edge effect. Ovigerous females were recorded in all months except in May 2011, suggesting continuous reproduction and the mean fecundity was 145 ± 121 embryos. There was a significant correlation between carapace width and number of embryos. Although the study area showed some signs of disturbance caused by anthropogenic action, the population of P. armatus was stable in its natural environment.