Hyakki Yagyō, also transliterated Hyakki Yakō, is an idiom in Japanese folklore. Sometimes an orderly procession, other times a riot, it refers to a parade of thousands of supernatural creatures known as oni and yōkai that march through the streets of Japan at night. As a terrifying eruption of the supernatural into the real world, it is similar to the concept of pandemonium in English.

Yōkai are a class of supernatural entities and spirits in Japanese folklore. The word yōkai is composed of two kanji characters that both mean "suspicious, doubtful", and while it may be regarded as a loanword from the Chinese term yaoguai, the word yōkai has taken on multiple different meanings particular to a Japanese context.