Metal nanoparticles (NPs) colloids provide a unique combination of optical, magnetic, electronic, and thermal properties, finding applications in various fields ranging from optoelectronics to cancer therapy. The chemical synthesis of such NPs has been refined to a high degree of control of crystallinity, shape, and size distribution. Among them, gold NPs are of particular interest because they host plasmonic resonances that enhance their interactions with light. Their size and shape impose the resonance wavelengths, which, depending on the application, must be tunable. For this, the synthesis processes are generally adapted on a case-by-case basis, preventing the development of a generic recipe that can cover a wide range of applications. We present here a different approach and propose to modify the size distribution of postsynthesis NPs by laser irradiation in the presence of a gold salt to tune their resonance wavelengths. Plasmonic resonances driven by femtosecond laser irradiation are used to tune the redox activity of AuCl4– ions in the presence of gold nanorods, which leads to a reshaping of NPs. All chemical intermediates and products are monitored by in situ UV–visible spectroscopy and ex situ electron microscopy analysis and then compared to 3D simulations in order to unveil the underlying photochemical processes. As such, these results not only pave the way for the fine-tuning of the plasmon resonance of gold colloids over a wide spectral range, which can have an impact on many application areas such as plasmonic colors, sensors, or absorbers but also feed into the very active field of plasmonic photocatalysis.