热力学平衡，简称热平衡，指一个热力学系统在没有外界影响的条件下，系统各部分的宏观属性（如物质的量、能量、体积等）在长时间内不发生任何变化的状态。

热平衡是热力学中的一个基本实验定律，其重要意义在于它是科学定义温度概念的基础，是用温度计测量温度的依据。

所以热平衡定律如第一、第二定律一样也是热力学中的基本实验定律，其重要性不亚于热力学第一、第二定律，但由于人们是在充分认识了热力学第一、第二定律之后才看出此定律的重要性，故英国著名物理学家拉尔夫·福勒称它为热力学第零定律。

Thermodynamic equilibrium is an axiomatic concept of thermodynamics. It is an internal state of a single thermodynamic system, or a relation between several thermodynamic systems connected by more or less permeable or impermeable walls. In thermodynamic equilibrium there are no net macroscopic flows of matter or of energy, either within a system or between systems. In a system in its own state of internal thermodynamic equilibrium, no macroscopic change occurs. Systems in mutual thermodynamic equilibrium are simultaneously in mutual thermal, mechanical, chemical, and radiative equilibria. Systems can be in one kind of mutual equilibrium, though not in others. In thermodynamic equilibrium, all kinds of equilibrium hold at once and indefinitely, until disturbed by a thermodynamic operation. In a macroscopic equilibrium, almost or perfectly exactly balanced microscopic exchanges occur; this is the physical explanation of the notion of macroscopic equilibrium.