The development of efficient, low-cost, and stable bifunctional catalysts is necessary for renewable energy storage and conversion, but it remains a challenge. Herein, we first report a novel strategy to develop WO3·nH2O (n = 0.33, 1.00, or 2.00) as a highly active and durable bifunctional catalyst for the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) in acidic media by controlling the degree of hydration. The content of solvated water molecules in WO3·nH2O can be precisely controlled by selectively using ethylenediaminetetraacetic acid or dl-malic acid for room-temperature precipitation synthesis. Structural flexibility associated with water solvation in WO3·nH2O leads to excellent bifunctional catalytic activity as well as durability in acidic media. The bifunctional catalytic mechanism of WO3·nH2O is mainly attributed to spontaneous partial dehydration during electrolysis, resulting in simultaneous formation of active phases for HER and ORR, respectively. Copyright © 2020 American Chemical Society.