在热力学之中，孤立系统（或孤悬系统）是指一个完全不与外界交换能量或质量的系统。任何能量或质量都不能进入或者离开一个孤立系统，只能在系统内移动。

除了把整个宇宙视为一体之外，孤立系统并不存在于现实之中。但是，在一定次数内，有些真实系统的行为近乎于孤立系统。因而，孤立系统的概念可以作为真实情况的一个近似模型。在创建以数学模型描述一些自然现象时，孤立系统是个可被接受的模型。

可被近似于孤立系统的模型包括：

在检证热力学第二定律中的熵增加的假设时，波兹曼所用的方程序便假设了系统是孤立的。

In physical science, an isolated system is either of the following:

Though subject internally to its own gravity, an isolated system is usually taken to be outside the reach of external gravitational and other long-range forces.

This can be contrasted with what (in the more common terminology used in thermodynamics) is called a closed system, being enclosed by selective walls through which can pass energy as heat or work, but not matter; and with an open system, which both matter and energy can enter or exit, though it may have variously impermeable walls in parts of its boundaries.