热力学系统（英语：Thermodynamic system）是指用于热力学研究的有限宏观区域，是热力学的研究对象。它的外部空间被称为这个系统的环境。一个系统的边界将系统与它的外部隔开。这个边界既可以是真实存在的，也可以是假想出来的，但必须将这个系统限制在一个有限空间里。系统与其环境可以在边界进行物质，功，热或其它形式能量的传递。而热力学系统可以从它的边界（或边界的一部分）所允许的传递类型进行分类。

热力学系统有一系列的状态函数，比如体积，压强，温度等。这些量都是可以通过实验测量的宏观量。这些量的数值共同决定这个系统的热力学状态。一个热力学系统的状态函数通常存在一个或多个函数关系。这些关系可由状态方程表述。平衡热力学不涉及对这些状态函数的通量的研究。因为由热力学平衡的定义可以自然得到，这些函数的通量的值为零。当然，平衡热力学可能会涉及使通量不为零的过程，但在热力学过程进行前，这些过程必需停止。非平衡热力学允许状态函数通量不为零。通量不为零表示在系统和它的环境间存在物质，能量或熵等的传递。

A thermodynamic system is the material and radiative content of a macroscopic volume in space, that can be adequately described by thermodynamic state variables such as temperature, entropy, internal energy and pressure. Usually, by default, a thermodynamic system is taken to be in its own internal state of thermodynamic equilibrium, as opposed to a non-equilibrium state. The thermodynamic system is always enclosed by walls that separate it from its surroundings; these constrain the system. A thermodynamic system is subject to external interventions called thermodynamic operations; these alter the system's walls or its surroundings; as a result, the system undergoes thermodynamic processes according to the principles of thermodynamics. (This account mainly refers to the simplest kind of thermodynamic system; compositions of simple systems may also be considered.)