A photon-to-thermal energy conversion nanosystem based the near-infrared irradiation of one-dimensional gold nanoparticles (nanorods) is highly efficient and tunable to the incident wavelength. Using ambient photothermal detection, we observed a temperature rise of ca. 30 degrees C upon irradiating an aliquot of an aqueous nanoparticle suspension with a laser for 5 s. The temperature can be elevated even higher by embedding the particles into a poorly thermally conducting solid medium. The illuminated area of a sample containing nanorod particles embedded in a polyurethane matrix can be heated to > 100 degrees C upon irradiation for 1 min. This optothermal conversion efficiency can be turned on selectively by tuning the wavelength to match that of the surface plasmon resonance of the particles. This specificity, with respect to the wavelength of the incident light, makes these highly efficient, particle-based, optothermal nanoconvertors suitable for potential use in multicolor detection on biochips and related sensors and as ideal contrasting agents for optoacoustic biomedical imaging applications.