Abstract Thermochromic smart windows and radiative coolers are two passive cooling technologies, whose adoption as windows and roofs, respectively, is feasible for building energy-saving. However, to the authors' knowledge, the investigation of annual energy performance incorporating both techniques is scarce at the time of writing. Therefore, a passive hybrid system involving both technologies is proposed in this study. A perovskite thermochromic smart window and three different radiative coolers were chosen based on their superior performance. The energy performance of the passive hybrid system in a prototypical medium-sized office building was simulated using EnergyPlus and the results were rigorously analyzed. Both thermochromic smart window and radiative cooler could reduce total energy consumptions by up to 10.6% and 23.0%, respectively, regardless of building's year of completion, while the synergic system saved up to 32.0%. Among the chosen cities of various climates, thermochromic smart windows and radiative coolers perform better in cities where cooling demand dominates. The west- and east-facing thermochromic smart windows could mitigate more energy usage in contrast to the other orientations. If this passive hybrid system can be offered at a reasonable cost, the technology is likely to be a viable energy-efficient option for buildings.