The Mariko Aoki phenomenon (青木まりこ現象, Aoki Mariko genshō) is a Japanese expression referring to a sudden urge to defecate that is felt upon entering bookstores. The phenomenon is named after Mariko Aoki, a woman who described the effect in a magazine article published in 1985. According to Japanese social psychologist Shōzō Shibuya, the specific causes that trigger a defecation urge in bookstores are not yet clearly understood. There are also some who are skeptical about whether such a peculiar phenomenon really exists at all, and it is sometimes discussed as one type of urban myth.

The series of processes through which being in a bookstore leads to an awareness of a defecation urge is something that cannot be explained from a medical perspective as a single pathological concept, at least at present. According to a number of discussions on the topic, even if it can be sufficiently found that this phenomenon actually exists, it is a concept that would be difficult to be deemed a specific pathological entity (such as a "Mariko Aoki disease", for example).