This study investigates the acute and sub-chronic toxicity of the emerging antibiotic, sulfamethoxazole (SMX) on the tropical freshwater microalga Chlorella sp. under laboratory condition. An acute exposure for 96 h and a sub-chronic exposure for 10 days were conducted to explore the adverse effects, utilizing growth inhibition, pigment content, and cell diameter as endpoints. The calculated half maximal (50 %) effective concentration (EC₅₀) values for SMX after 24 h and 96 h were estimated to be 0.56 mg/L and 0.41 mg/L, respectively. Sub-chronic exposure unveiled a pronounced inhibitory effect of SMX on algal growth, with significant and dose-dependent increases in growth inhibition observed. Across all treatments, exposure to SMX resulted in growth inhibition and a decline in chlorophyll-a (Chl-a) concentration in the test algae. Notably, at a concentration of 1 mg/L, SMX completely inhibited the growth of Chlorella sp. and reduced Chl-a content by up to 98 %. High concentrations of SMX (0.5 and 1.0 mg/L) led to a decrease in cell diameter in the tested algae. This study contributes valuable insights into the acute and sub-chronic toxic effects of environmentally relevant concentrations of antibiotics on tropical freshwater microalgae.