In psychology, the Stroop effect is a demonstration of cognitive interference where a delay in the reaction time of a task occurs due to a mismatch in stimuli.

The effect is named after John Ridley Stroop, who first published the effect in English in 1935. The effect had previously been published in Germany in 1929 by other authors. The original paper has been one of the most cited papers in the history of experimental psychology, leading to more than 700 Stroop-related articles in literature.