The generation of reactive oxygen species by gold nanorods under ultrashort-pulsed irradiation is investigated at the single-nanoparticle level. Near-infrared femtosecond pulses induce the production of hydroxyl radicals in a hydrogel medium through a plasmon-assisted, polarization-controlled, multiphoton process. Three - photon luminescence is also observed, allowing a potential replacement for photodynamic agents combined with nonlinear imaging properties.