The distribution of phytoplankton in a coastal gradient located in the pelagic area of the Maranhão continental shelf was analyzed. Samples were collected in six bimonthly campaigns with seven sampling stations from November 2013 to September 2014. Simultaneously, environmental parameters were obtained, such as rainfall, salinity, conductivity, pH, dissolved oxygen, suspended particulate matter (SPM), water temperature, water transparency, nitrite, phosphate and silicate concentrations. The values of SPM showed a decreasing profile toward the ocean. The nutrients showed a clear seasonal cycle. The pH maintained alkaline during all the study period. There was an increase in electrical conductivity in direction of the ocean, the same pattern was observed for salinity. Chlorophyll-a concentration presented the highest values during the dry season and the lowest in the rainy one. The total chlorophyll-a also showed a decreasing profile toward the ocean in both dry and rainy season and was very low if compared with other studies in northern Brazil. The scenario observed in this study showed a clear seasonal cycle, with the highest contribution of the phytoplankton in the dry period; also it was observed the predominance of the Bacillariophyta division over the other identified divisions found, based on the obtained qualitative data.