Mesoscale eddies are ubiquitous oceanic structures that play pivotal roles in planktonic ecosystems. In this paper, we present observational evidence of the impact of a mesoscale cyclonic eddy on the phytoplankton biomass, measured by chlorophyll-a concentration, in the Bay of La Paz, Gulf of California, using information gathered during a multidisciplinary research cruise in the summer (August) of 2017. A CTD rosette system was used to record high-resolution hydrographic readings and to take water samples for nutrients and spectrophotometric determination of chlorophyll-a concentrations, throughout a grid of stations covering the whole area. The results showed that a mesoscale cyclonic eddy, with a diameter of ∼20 km and a maximum velocity of 0.95 m s^–1 at its periphery, had established from the surface to a depth of 100 m, inducing differences in the hydrographic properties of the water column. The phytoplankton biomass distribution was closely related to the circulation pattern, with the highest chlorophyll-a concentration (>3 mg m^–3) observed in the center of the eddy, in a layer between 20 and 30 m depth, with absolute maximum values located at 20 m depth as a consequence of nutrient pumping. The presence of mesoscale eddies in the Bay of La Paz has been described previously, but their impact on the phytoplankton biomass and its three-dimensional variability has not been quantified systematically. Therefore, further studies that document the effects of these eddies on phytoplankton biomass are needed to better understand highly productive marine areas, such as the Bay of La Paz, and to propose conservation and management strategies.