Titanium dioxide nanoparticles (TiO₂ NPs) present a high interest as an excellent antibacterial agent against pathogenic microorganisms. Currently, the massive proliferation of potentially toxic cyanobacteria in the water of dams presents a threat to the public health of these water consumers. This study was maintained to evaluate the antibacterial activity of TiO₂ NPs against the cyanobacteria « Microcystis sp.» responsible for toxic blooms in the freshwater reservoirs intended for drinking water production. Three concentrations of TiO₂ NPs (150 mg.l^-1¸ 300 mg.l^-1 and 600 mg.l^-1) were tested in the laboratory on two strains of Microcystis sp. (S1¸ S2) over a period of eight days. In parallel, three biotic variables were measured: Microcystis sp. cells density (cell.ml^-1), Chlorophyll (a) (µg.l^-1), and Phycocyanin (µg.l^-1) content every 48 hours all along the experimental period. A remarkable decrease in cell densities was registered, recording decreased values from 77490 to 39091 cell.ml^-1 for S1 and from 2801 to 0 cell.ml^-1 for S2 with the concentration of 600 mg.l^-1. Additionally, the chlorophyll (a) content fell from 45 to 8 µg.l^-1 for S1, and from 34 to 0 µg.l^-1 for S2. Similarly, treatment with TiO₂ NPs caused the degradation of phycocyanin from 200 to 65 µg.l^-1 for S1 and from 200 to 0 µg.l^-1 for  S2 with the highest concentration of these nanoparticles (600 mg.l^-1). TiO₂ NPs exhibit a very important antibacterial effect against Microcystis. sp. Thus, TiO₂ NPs can be used in the future to reduce the high biomasses of cyanobacteria during the process of tap water production.