Janus nanoparticles containing plasmonic materials have attracted great attention in the thermoplasmonic community due to their potential for applications in thermoelectronics or biomedicine. A significant number of thermoplasmonic applications rely on the heating of nanostructures by using pulsed excitation lasers. The heating generated in these nanostructures is often transferred to other regions via material–fluid interfaces. This heat transfer dynamics is controlled by the interfacial thermal conductance. In this work, we investigate the impact of the interfacial thermal conductance on the thermal relaxation of metal–polymer Janus nanoparticles that generate directional heating under pulsed illumination. We show that neglecting the temperature dependence of the thermophysical properties results in an overestimation of the temperature of the nanoparticle. A gold/polymer semishell nanostructure was used as an example not only to illustrate the aforementioned effects but also to show it as a reliable nanoheater candidate for photothermal therapies, capable of offering a remarkable temperature increment and presenting directional heating. The model we developed here can be applied to any type of nanoarquitecture, showing this work as a powerful tool for topics beyond photothermal therapies that can contribute to the development of novel structures able to control heat on the nanoscale.