The use of P25 TiO2 NPs in consumer products, their release, and environmental accumulation will have harmful effects on the coastal ecosystems. The sensitivity to TiO2 NPs may vary depending on the structural property and physiological mechanism of algal species. Therefore, the present study investigates the differences in sensitivity of two marine algae, Dunaliella salina and Chlorella sp., towards P25 TiO2 NPs. Among the two species, Chlorella sp. was more sensitive to TiO2 NPs than Dunaliella salina. The different working concentrations of TiO2 NPs, 0.1, 1, and 10 mg L−1, were selected based on the EC50 value. The EC50 value of TiO2 NPs for Dunaliella salina was found to be 1.8 and 13.3 mg L−1 under UV-A and dark conditions, respectively. The EC50 value of TiO2 NPs for Chlorella sp. was found to be 1.6 and 5.0 mg L−1 under UV-A and dark conditions, respectively. The decrease in cell viability was significantly higher for Chlorella sp. compared to Dunaliella salina at all concentrations except 0.1 mg L−1. The cellular viability data was in correlation with the oxidative stress markers such as total ROS and LPO. A concentration-dependent increase in ROS and lipid peroxidation was noted under UV-A exposure, which was higher in Chlorella sp. compared to Dunaliella salina. The decrease in the SOD activity with NP concentration was more in Dunaliella salina than Chlorella sp. under both conditions, whereas Chlorella sp. showed increased CAT activity with increasing concentration. The uptake of TiO2 NPs was more in Chlorella sp. than Dunaliella salina.