Silver TiO₂ nanocomposites were synthesized by two methods: silver UV photodeposition (Ag/P25) and a sol–gel process incorporating silver nitrate during the titanium alkoxide gelling step (TiO₂-Ag). The obtained nanocomposites were characterized by means of XRD, N₂ adsorption, XPS, and DRS. All the silver-TiO₂ semiconductors have anatase as the principal crystalline TiO₂ phase and the average band gap was found in the edge of the visible–ultraviolet region (3.26eV). According to XPS and HAADF-STEM studies, highly dispersed silver nanoparticles were found on the titania surface as Ag°. The photocatalytic effects of the silver TiO₂ nanocomposites on the killing of marine algae were examined by using either Amphidinium carterae (red tide) or noxious Tetraselmis suecica (green tide) as a probe. After 1h under UV irradiation, both harmful algae were inactivated by the silver nanocomposites. The fatal damages to these microorganisms induced by the TiO₂-Ag sol–gel semiconductor occurred faster than those promoted by the Ag/P25 and TiO₂ sol–gel isolated supports. In addition to the biocide properties of silver in aqueous medium, the silver nanoparticles acted as electron traps, retarding electron–hole recombination, which enhanced the photocatalytic activity.