Increased utilization of titanium dioxide nanoparticles (TiO2 NPs) for commercial as well as industrial purposes resulted in the accumulation of nanoparticles in the marine system. Microplastics being an emerging secondary pollutant in the marine ecosystem have an impact on the toxic effects of TiO2 NPs which has not been evaluated up to date. So it is important to assess the toxic effects of both these pollutants on the marine environment. The present study examines the impact of differently functionalized microplastics on the toxic effects of P25 TiO2 NPs to marine algae Chlorella sp. The tendency of nanoparticles to undergo aggregation in artificial seawater was observed with increase in time. The median effective concentration for TiO2 NPs was found to be 81 μM which indicates higher toxic effects of NPs toward algae. In contrast, microplastics irrespective of their difference in functionalization had minimal toxic effect of about 15% at their higher concentration tested, 1000 mg L−1. Plain and aminated polystyrene microplastics enhanced the TiO2 NPs toxicity which was further validated with oxidative stress determination studies like reactive oxygen species and lipid peroxidation assays. Negatively charged carboxylated polystyrene microplastics decreased the TiO2 NPs toxicity with possible hetero-aggregation between TiO2 NPs and microplastics in the system. The toxicity data obtained for the mixture was further corroborated with Abbott's mathematical model. © 2018 Elsevier B.V.