Pollutants such as n-TiO2 and tetracycline enter the marine environment through various sources starting from their production until disposal. Hence, it is vital to determine the interactive effect of one pollutant with the other when they coexist in the environment. In the present study, the effect of antibiotic – tetracycline (TC) on the toxicity of P25 n-TiO2 was studied with marine microalgae, Chlorella sp. The impact of TC (1 mg L−1) on five different concentrations of n-TiO2 (0.25, 0.5, 1, 2 and 4 mg L−1) under both visible and UV-A illuminations was evaluated. Effective diameter of n-TiO2 in ASW at 0th h increased from 690.69 ± 19.55 nm (0.25 mg L−1) to 1183.04 ± 37.10 nm (0.25 mg L−1 + 1 mg L−1) and 971.51 ± 14.61 nm (4 mg L−1) to 1324.12 ± 11.59 nm (4 mg L−1 + 1 mg L−1) in presence of TC. A significant increase in the toxicity of 4 mg L−1 n-TiO2 upon the addition of TC (68.16 ± 0.37% under visible and 80.21 ± 0.3% under UV-A condition) was observed. No significant difference in toxicity was observed between visible and UV-A illuminations. Further the toxicity data was corroborated through the measurement of oxidative stress and antioxidant enzyme activities. Independent action model showed antagonistic effect for lower concentrations of n-TiO2 and additive effect for higher concentrations of n-TiO2 when present in mixture with TC under both illuminations. For the higher mixture concentration of 4 mg L−1 n-TiO2 and 1 mg L−1 TC, the percentage TC removal was about 55.29% and 30% and the corresponding TOC removal was found to be 54.29% and 31.04% under visible and UV-A illuminations respectively. The site of ROS generation in Chlorella sp. was identified with electron transfer chain inhibitors. Both mitochondria and chloroplast acted as the site for the ROS generation in Chlorella sp. The SEM images of the algal cells upon exposure to n-TiO2 and mixture revealed the aggregation of cells and distortion of cell membrane. © 2019 Elsevier Inc.