White light scattering spectra based on dark-field microscopy is a powerful tool for studying localized surface plasmon resonances of single noble metal nanoparticles and for developing their applications in sensing and biomedical therapies. Here we demonstrate the steady-state scattering spectral changes of a single gold nanoparticle under continuous laser heating. The experimental peak shifts allowed the estimation of the particle temperature in the range of 300–700 K with an accuracy of ±20 K on the basis of a spectral calculation exploiting Mie theory. For single gold nanoparticles supported on a glass substrate, progressive red shifts were observed in air, whereas blue shifts were observed in water and glycerol with increasing temperature. The medium has strong influence on peak shifts because of distance-dependent nanoscale medium heating: the shifts strongly depend on the magnitude of the temperature coefficients of a medium refractive index. The laser power-dependent behavior of peak shifts revealed the onset of surface melting occurring at 550–600 K regardless of the medium. Furthermore, experimental shifts also suggested that surface liquid layer grows in thickness with increasing temperature until the whole particle melts: this model has been proposed theoretically as a liquid nucleation and growth hypothesis. Therefore, we showed that the scattering spectral shifts represent an effective measure for the laser-induced morphological alterations of a gold nanoparticle and the nanoscale heating of a medium surrounding the NP.