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Abstract Metal oxysulfides are an emerging group of sensitive materials for high‐performance NO2 sensing owing to their room‐temperature operation capacity, excellent selectivity of NO2, the part‐per‐billion‐leveled limit of detection, as well as high stability against the ambient environment. Here, the room‐temperature NO2 sensing performances of zinc oxysulfide 3D micro‐self‐assembly composed of ultrathin nanoflakes are investigated. The combined hydrothermal–annealing approach is applied to first synthesize zinc sulfide micro‐self‐assembly and then transform it into zinc oxysulfide in a controlled environment. As a result, the majority of S atoms in zinc sulfide are replaced with O atoms, leading to the crystal structure variation from cubic/hexagonal to an orthorhombic configuration. Simultaneously, the corresponding optical bandgap is reduced from ≈3.6 – 3.8 eV to ≈1.92 eV, enabling the visible light harvesting capability. The sensor demonstrates a fully reversible and repeatable sensing response toward 1.26 ppm NO2 gas at room temperature with a response magnitude of ≈2.27 under the 460 nm excitation, a limit of detection (LOD) of 294.8 part‐per‐trillion (ppt), and almost an order of magnitude larger compared to other commonly used gas species. This work demonstrates the great potential of the metal oxysulfide framework for developing next‐generation room‐temperature NO2 gas sensors.