Hypochlorous acid (HClO, HOCl, or ClHO) is a weak acid that forms when chlorine dissolves in water, and itself partially dissociates, forming hypochlorite, ClO⁻. HClO and ClO⁻ are oxidizers, and the primary disinfection agents of chlorine solutions. HClO cannot be isolated from these solutions due to rapid equilibration with its precursor, chlorine.

Because of its strong antimicrobial properties, the related compounds sodium hypochlorite (NaClO) and calcium hypochlorite (Ca(ClO)₂) are ingredients in many commercial bleaches, deodorants, and disinfectants. The white blood cells of mammals, such as humans, also contain hypochlorous acid as a tool against foreign bodies.

Like many other disinfectants, hypochlorous acid solutions will destroy pathogens, such as COVID-19, adsorbed on surfaces. In low concentrations, such solutions can serve to disinfect open wounds.