Glyphosate is an active ingredient in increasingly used herbicides. Runoff and aerosolized glyphosate can enter river and lake systems leading to selective pressure on different species, including harmful algal bloom (HAB)-forming plankton. We examined the effects of glyphosate on the growth and photophysiology of the cyanobacterium Microcystis aeruginosa (UTEX-LB-2386). We grew M. aeruginosa in the presence of environmentally relevant concentrations of glyphosate ranging from (0 - 10 µg/L), and measured growth using spectrophotometry. Photophysiology of cells in exponential growth was analyzed using PAM-fluorometry. Assessments included in-vivo chlorophyll (F0), the maximum quantum yield of Photosystem II (QYmax, a proxy for photosynthesis efficiency and cell stress), and relative electron transport rates across PSII (a proxy for photosynthesis). Growth rates ranged from (0.05 to 0.07), and QYmax values ranged from (0.45 to 0.55). However, no measurable dose-response relationship was observed across all glyphosate concentrations tested. The lack of inhibition in M. aeruginosa growth or photophysiology at environmentally relevant levels of glyphosate suggest that these glyphosate levels could potentially provide a selective advantage to this bloom-forming species in contaminated freshwater ecosystems. Furthermore, given that the glyphosate levels tested were significantly below EPA thresholds for safe drinking water, our work also highlights the need to integrate robust contaminant monitoring into HAB-monitoring strategies - especially in watersheds with high agricultural activity.