Urf (العرف) is an Arabic Islamic term referring to the custom, or 'knowledge', of a given society. To be recognized in an Islamic society, Urf must be compatible with the Sharia law. When applied, it can lead to the deprecation or inoperability of a certain aspect of fiqh فقه (Islamic jurisprudence).

`Urf is a source of rulings where there are not explicit primary texts of the Qur'an and Sunnah specifying the ruling. `Urf can also specify something generally established in the primary texts.

In some countries such as Egypt, marriage, the Urfi way, means to get married without official papers issued by the state (Zawag Urfi:زواج عرفي).^{[citation needed]} This type of marriage is valid and recognized;^{[by whom?]} see: common law marriage.

The term urf, meaning "to know", refers to the customs and practices of a given society. Although this was not formally included in Islamic law, the Sharia recognizes customs that prevailed at the time of Muhammad but were not abrogated by the Qur'an or the tradition (called "Divine silence"). Practices later innovated are also justified, since Islamic tradition says what the people, in general, consider good is also considered as such by Allah (see God in Islam). According to some sources, urf holds as much authority as 'ijma (consensus), and more than qiyas (legal reasoning by analogy). Urf is the Islamic equivalent of "common law".