AbstractDaytime radiative cooling provides an eco‐friendly solution to space cooling with zero energy consumption. Despite significant advances, most state‐of‐the‐art radiative coolers show broadband infrared emission with low spectral selectivity, which limits their cooling temperatures, especially in hot humid regions. Here, an all‐inorganic narrowband emitter comprising a solution‐derived SiO_xN_y layer sandwiched between a reflective substrate and a self‐assembly monolayer of SiO₂ microspheres is reported. It shows a high and diffusive solar reflectance (96.4%) and strong infrared‐selective emittance (94.6%) with superior spectral selectivity (1.46). Remarkable subambient cooling of up to 5 °C in autumn and 2.5 °C in summer are achieved under high humidity without any solar shading or convection cover at noontime in a subtropical coastal city, Hong Kong. Owing to the all‐inorganic hydrophobic structure, the emitter shows outstanding resistance to ultraviolet and water in long‐term durability tests. The scalable‐solution‐based fabrication renders this stable high‐performance emitter promising for large‐scale deployment in various climates.