Stop bath is the second of three chemical baths usually used in processing traditional black-and-white photographic films, plates, and paper. The sequence is: developer, stop bath, fixer. A simple rinse can be used between developer and fixer, but the development process continues for an indefinite and uncontrolled period of time during the rinsing.

Stop bath usually consists of some concentration of acetic acid. By neutralizing the alkalinity of the developer, stop bath stops the development process almost instantly and thus provides more precise control of the development time. It also cuts overall processing time, because the required immersion time in the stop bath—typically fifteen seconds—is much shorter than the time required for an adequate plain-water rinse.

Stop bath accounts for the characteristic vinegar-like odor of the traditional dark room, and in its concentrated form can cause chemical burns. For indicator stop bath, the pH indicator bromothymol violet is used to determine when the solution has become too alkaline to use.