Urban lakes and lagoons are bodies of water that are subject to environmental impacts and their degree of alteration can be measured by studying the phytoplankton, sensitive organisms that respond quickly to ecological changes and can detect any change in water quality. The aim of this work is to know the phytoplankton richness of the periurban lagoon Don Tomás, its spatio-temporal distribution and to determine the current trophic state. Seventy-eight taxa belonging to the Classes Cyanophyceae (20), Chlorophyceae (44), Bacillariophyceae (10) and Euglenophyceae (4) were determined, with higher specific richness in spring-summer. The composition of the phytoplankton taxocenosis observed in Don Tomás Lagoon is characteristic of eutrophicated lagoons.