Hydrogen (H2) is recognized as a vital solution for constructing a sustainable energy society owing to its abundance, high efficiency, and lack of pollution. However, H2 is the lightest gas on earth and can embrittle metals, leading to hydrogen leakage. Because hydrogen has a wide flammability range of 4–74 vol.% in air and low ignition energy, it is crucial to identify hydrogen leakages quickly and efficiently to ensure safety. Although various hydrogen sensors have been developed, gasochromic hydrogen sensors, which are based on color changes caused by gasochromic nanomaterials, show great potential for use in the future hydrogen-based world because they are inexpensive, change color very quickly, and work at room temperature. This technical paper focuses on gasochromic hydrogen sensors. First, gasochromic mechanisms based on WO3 nanomaterials are introduced, and three aspects of state-of-the-art research, WO3 nanomaterials with novel structures, noble metals to promote reactions, and flexible fabrication, are presented. Research on the former two aspects aims to develop high-performance gasochromic nanomaterials, and research on flexible fabrication, i.e., transforming nanomaterials into potential hydrogen sensors, is reviewed here for the first time. In addition, other non-WO3-based gasochromic systems are briefly reviewed. Finally, this technical paper provides a brief perspective for future research.