The gasochromic effect has important applications in energy savings, for example, in a smart window. This study reveals evidence of the existence of localized water molecules in colored WO3 nanowires and their important role in the gasochromic effect. Such water molecules can be moved out from the nanowires. The coexistence of the water molecules and oxygen vacancies in nanowires leads to a defect band in the band gap. Coloration is attributed to absorptions of photons involving this defect band. These findings deepen the understanding of the physical mechanism underlying the gasochromic effect of WO3.