An optoacoustic (OA) sensor was designed, fabricated, and used to detect spherical gold nanoparticles (NPs) in diluted suspensions. The sensor, operating in the backward mode, was designed to measure signals from microscopic volumes of nanoparticulate suspensions in water. Thermal nonlinearity was observed in the course of OA signal generation. The irradiation of a microvolume of gold nanoparticles at the wavelength matching the peak of their plasmon resonance absorption gives rise to a multitude of thermomechanical processes, including heating of NPs below the critical temperature of water (374 K). The thermal diffusion from nanoparticles to water takes place; however, formation of vapor nanobubbles is avoided. As a result, a specific acoustic signal is produced exhibiting nonlinear behavior with respect to the incident laser pulse energy. The optoacoustic profile of the laser-induced signal generated in a thin layer of highly diluted suspensions of gold nanospheres was examined, thereby providing a basis for a method for detection of metal nanoparticles with high sensitivity.