在热力学里，内能（internal energy）是热力学系统内两个具状态变量之基本状态函数的其中一个函数。内能是指系统所含有的能量，但不包含因外部力场而产生的系统整体之动能与位能。内能会因系统能量的增损而随之改变。

系统的内能可能因(1)对系统加热、(2)对系统作功，或(3)添加或移除物质而改变。当系统内有不可穿透的墙阻止物质传递时，该系统称之为「封闭系统」。如此一来，热力学第一定律描述，内能的增加会等于增加的热量加上环境对该系统所作的功。若该系统周围的墙不能传递物质与能量，则该系统称之为「孤立系统」，且其内能会维持定值。

In thermodynamics, the internal energy of a system is the energy contained within the system, excluding the kinetic energy of motion of the system as a whole and the potential energy of the system as a whole due to external force fields. It keeps account of the gains and losses of energy of the system that are due to changes in its internal state.