妖怪，指草木或者动物等改变成为的精怪，也指怪异、反常的事物与现象。

妖怪通常存在于人类想像与传说之中，难以运用科学方法证明其真伪。研究这方面的学问，称之为妖怪学，包含在民俗学、民族学、文化人类学、语言社会学等学科研究里，虽不属于独自发展的学问，亦是一门有系统的学说。

有关妖怪的起源，已经无从稽考。被称为妖怪之列的，大多给人的印象都是鬼异邪祟，倾向做坏事为多的一类；有些则给予人神秘莫测，不知是敌是友的感觉。有些则是被当做神一样尊敬，令人崇拜。

在欧洲语言中没有完全对应于妖怪的词汇，仅有意义相近的词汇，例如英语的 monster（怪物）、gost、spook（鬼）、giant（巨人）、undead（不死生物）、devil（恶魔）、demon、fiend、evil spirit（邪灵）、elf（精灵）、goblin（哥布林）、bogy、fairy（小仙子）。

Yōkai(妖怪, ghost, phantom, strange apparition) are a class of supernatural monsters, spirits and demons in Japanese folklore. The word yōkai is made up of the kanji for "bewitching; attractive; calamity;" and "spectre; apparition; mystery; suspicious". They can also be called ayakashi(妖), mononoke(物の怪), or mamono(魔物). Yōkai range eclectically from the malevolent to the mischievous, or occasionally bring good fortune to those who encounter them. Often they possess animal features (such as the Kappa, which is similar to a turtle, or the Tengu which has wings), other times they can appear mostly human, some look like inanimate objects and others have no discernible shape. Yōkai usually have a spiritual supernatural power, with shapeshifting being one of the most common. Yōkai that have the ability to shapeshift are called obake.