禁食（fasting）是指个人有意识地停止进食某些或所有食物，甚至饮料。禁食亦包括禁止食用某些特定食物（例如肉类、用不同方式处理的食物）。

禁食行为存在于很多宗教的信仰准则中。其目的为准备节庆，反省自身，或教导信徒节制贪欲，进求更高层次的生活，通常被称为斋戒。

有些宗教禁止教徒使用特定的食品，有些宗教限制禁食期长短、可进食的时间、份量、特定处理方式、戒肉等。

禁止教徒食某些肉类（如猪肉、狗肉、无鳞鱼类）和未按教规处理的食物。穆斯林在斋月的白天不准进食。

摩西﹑耶稣均曾进行四十天禁食。

复活节前禁食：四旬期/大斋期禁食数星期以至只禁食数天。有些天主教徒会将餐数限制为两餐，戒肉或限制餐量。

Fasting is primarily a movement of willing abstinence or reduction from some or all food, drink, or both, for a period of time. An absolute fast (dry fasting or respirationism) is normally defined as abstinence from all food and liquid for a defined period, usually a period of 24 hours, or a number of days. Water fasting allows drinking water but nothing else. Other fasts may be only partially restrictive, limiting particular foods or substances. The fast may also be intermittent in nature. Fasting practices may preclude intercourse and other activities as well as food.