Water electrolysis offers an ideal process for generating hydrogen, the most promising form of clean energy. Splitting water in acidic media has had limited progress to date because of the low anodic oxygen evolution reaction activity and serious catalyst degradation. Here, we report a bifunctional Ir catalyst stabilized by cucurbit[6]uril for water splitting in acidic media. We achieved a 10 mA/cm² overall water-splitting current density at only 1.56 V, which is among the highest activities reported. Moreover, our cell showed a high stability of at least 20 h of continuous operation at 5 mA/cm². Spectroscopic experiments and theoretical calculations revealed that the coordination interaction between Ir atoms and cucurbit[6]uril facilitates the controllable synthesis and stabilization of surface active IrO_x species, which leads to the excellent catalytic activity and stability observed.