焦耳定律或焦耳－冷次定律是定量说明传导电流将电能转换为热能的定律。1841年，英国物理学家詹姆斯·焦耳发现载流导体中产生的热量Q（称为焦耳热）与电流I的平方、导体的电阻R和通电时间t成比例。而在1842年时，俄国物理学家海因里希·楞次也独立发现上述的关系，因此也称为「焦耳－楞次定律」。

采用国际单位制时，焦耳定律的表达式为：

其中Q（热量）、I（电流）、R（电阻）、t（时间）、P（热功率）各量的单位依次为焦耳、安培、欧姆、秒和瓦特。
焦耳定律是设计电照明，电热设备及计算各种电气设备温升的重要公式。

Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor releases heat. The amount of heat released is proportional to the square of the current such that

This relationship is known as Joule's first law or Joule–Lenz law. Joule heating is independent of the direction of current, unlike heating due to the Peltier effect.