Chengyu (traditional Chinese: 成語; simplified Chinese: 成语; pinyin: chéngyǔ; lit. '[already] made/formed words/speech') are a type of traditional Chinese idiomatic expression, most of which consist of four characters. Chengyu were widely used in Classical Chinese and are still common in vernacular Chinese writing and in the spoken language today. According to the most stringent definition, there are about 5,000 chéngyǔ in the Chinese language, though some dictionaries list over 20,000. Chéngyǔ are considered the collected wisdom of the Chinese culture, and contain the experiences, moral concepts, and admonishments from previous generations of Chinese. Nowadays, chéngyǔ still play an important role in Chinese conversations and education. Chinese idioms are one of four types of formulaic expressions (熟语/熟語, shúyǔ), which also include collocations (惯用语/慣用語 guànyòngyǔ), two-part allegorical sayings (歇后语/歇後語 xiēhòuyǔ), and proverbs (谚语/諺語 yànyǔ).

They are often referred to as Chinese idioms or four-character idioms; however, they are not the only idioms in Chinese.