<p>Plasmonic nanoparticles can efficiently convert electromagnetic energy into heat by undergoing coherent electronic oscillations, known as localized surface plasmon (LSP). After optical absorption, the excited (hot) electrons decay, releasing energy via radiative and non-radiative processes. Moreover, a plasmon-mediated energy transfer to the medium has been demonstrated. This effect could lead to an effective reduction of the thermal resistance of the nanoparticle, facilitating the diffusion of heat, i.e., to an enhancement of the thermal diffusivity. The key factor is the precise matching of the excitation beam wavelength with the one corresponding to the peak of the localized surface plasmon resonance (LSPR). In this study, we report the measurement of the thermal diffusivity of Au nanofluids, whose nanoparticles were synthesized via a photochemical route, using the dual-beam mode-mismatched thermal lens technique. Our results show that there is a 25% increase in thermal diffusivity in nanofluids containing nanoparticles whose maximum absorption band of LSPR occurs at the excitation beam wavelength.</p>