The beach of Porto de Galinhas is socially and economically important because of the reef environment that attracts a large number of tourists. In order to contribute to the monitoring of the environmental quality of the area, which is subject to strong human pressure, phytoplankton composition was analysed in four fixed sampling station, during 4 months of the dry and rainy periods, at high and low tides and at diurnal spring tides. The phytoplankton community structure and the abiotic parameters varied according to seasonality. Phytoplankton biomass ranged from 0.42 to 5.66 mg m−3 and density ranged from 0.50 to 14.3 cell 103 l−1. A total of 192 taxa were identified, belonging to six divisions. The most representative taxa were Ochrophyta (64.6%), followed by Dinophyta (19.3%). Ostreopsis ovata, Trichodesmium erythraeum, Protoperidinium bispinum, Paralia sulcata, Thalassiosira leptopus and Fragilaria capucina were the most representative species of the environment. Ostreopsis ovata was the only highly frequent and dominant species. The Easterly Wave Disturbance contributed to the occurrence of a large number of freshwater species that were observed on the site. The environment was characterized as oligotrophic, tending toward mesotrophic in the rainy season. The high number of taxa associated with high evenness was indicative of environmental balance. Continental interference was perceived due to the variation of abiotic parameters and of the variation of the phytoplankton community structure.