Yōkai (妖怪; Japanese pronunciation: [joː.kai]) are a class of supernatural entities and spirits in Japanese folklore. The kanji representation of the word yōkai comprises two characters that both mean "suspicious, doubtful", and while the Japanese name is simply the Japanese transliteration or pronunciation of the Chinese term yāoguài (which designates similarly strange creatures), some Japanese commentators argue that the word yōkai has taken on many different meanings in Japanese culture, including referring to a large number of uniquely Japanese creatures.

Yōkai are also referred to as ayakashi (あやかし), mononoke (物の怪) or mamono (魔物). Some academics and Shinto practitioners acknowledge similarities within the seeming dichotomy between the natures of yōkai and most kami, which are generally regarded as relatively beneficent in comparison, and class the two as ultimately the same type of spirits of nature or of a mythological realm. Their behavior can range from malevolent or mischievous to benevolent to humans.

Yōkai often have animal-like features (such as the kappa, depicted as appearing similar to a turtle, and the tengu, commonly depicted with wings), but may also appear humanoid in appearance, such as the kuchisake-onna (口裂け女). Some yōkai resemble inanimate objects (such as the tsukumogami), while others have no discernible shape. Yōkai are typically described as having spiritual or supernatural abilities, with shapeshifting being the most common trait associated with them. Yōkai that shapeshift are known as bakemono (化け物) or obake (お化け).

Japanese folklorists and historians explain yōkai as personifications of "supernatural or unaccountable phenomena to their informants".^{[This quote needs a citation]} In the Edo period (1603 to 1868), many artists, such as Toriyama Sekien (1712-1788), invented new yōkai by taking inspiration from folktales or purely from their own imagination. Today, several such yōkai (such as the amikiri) are mistakenly thought to originate in more traditional folklore.

The concept of yōkai varies greatly throughout Japanese culture and historical periods; typically, the older the time period, the higher the number of phenomena deemed to be supernatural and the result of yōkai. According to Japanese ideas of animism, spirit-like entities were believed to reside in all things, including natural phenomena and objects. Such spirits possessed emotions and personalities: peaceful spirits were known as nigi-mitama, who brought good fortune; violent spirits, known as ara-mitama, brought ill fortune, such as illness and natural disasters. Neither type of spirit was considered to be yōkai.

One's ancestors and particularly respected departed elders could also be deemed to be nigi-mitama, accruing status as protective spirits who brought fortune to those who worshiped them. Animals, objects and natural features or phenomena were also venerated as nigi-mitama or propitiated as ara-mitama depending on the area.

Rituals for converting ara-mitama into nigi-mitama were performed, aiming to quell malevolent spirits, prevent misfortune and alleviate the fear arising from phenomena and events that otherwise had no explanation. The ritual for converting ara-mitama into nigi-mitama was known as the chinkon (鎮魂, lit. 'the calming of the spirits' or 'requiem'). Chinkon rituals for ara-mitama that failed to achieve deification as benevolent spirits, whether through a lack of sufficient veneration or through losing worshippers and thus their divinity, became yōkai.

Over time, phenomena and events thought to be supernatural became fewer and fewer, with the depictions of yōkai in picture scrolls and paintings beginning to standardize, evolving more into caricatures than fearsome spiritual entities. Elements of the tales and legends surrounding yōkai began to be depicted in public entertainment, beginning as early as the Middle Ages in Japan. During and following the Edo period, the mythology and lore of yōkai became more defined and formalized.