Fasād (Arabic: فساد /fa.saːd/), or fasaad, is an Arabic word meaning 'rottenness', 'corruption', or 'depravity'. In an Islamic context, it can refer to "spreading corruption on Earth" or "spreading mischief in a Muslim land", moral corruption against Allah, or disturbance of the public peace.

The spread of fasad is a major theme in the Quran, and the notion is often contrasted with islah ("setting things aright"). Classical Quranic commentators commonly interpreted "corruption in the land" as open disobedience against God or its result. In certain contexts, classical jurists took it to refer to the legal category of Hirabah, comprising armed assault, rape and murder. Some contemporary Muslims view destruction of the natural environment to be among the central meanings of verses referring to fasad.

The term has been used in the legal codes of the Islamic Republic of Pakistan and Islamic Republic of Iran. In Iran, laws referencing it have been used to prosecute or threaten political opposition figures.

The spread of fasad is a major theme in the Quran, and the notion is often contrasted with islah ("setting things aright").

Terms derived from the verbal root f-s-d appear in a number of Quranic verses. The verb afsad (to cause fasad) appears, for example, in chapter 2 (Al-Baqara), verse 11-12,

When it is said to them: "Make not mischief on the earth," they say: "Why, we only Want to make peace!" Of a surety, they are the ones who make mischief, but they realise (it) not. – Quran 2:11

Classical Quranic commentators commonly interpreted spreading "corruption in the land" or "mischief on the earth" as open disobedience against God or its result. Some contemporary Muslims view destruction of the natural environment to be among the central meanings of these verses.

In chapter 5 (Al-Ma'ida) of Qur'an, verse 33 contains the expression "to strive mischievously in the land" (yas'awna fi-l-ardi fasadan):