热力学第三定律是热力学的四条基本定律之一，其表述的是热力学系统的熵在温度趋近于绝对零度时趋于定值。而对于完整晶体，这个定值为零。

这一定律由瓦尔特·能斯特归纳得出和表述，因而又常被称为能斯特定理或能斯特假定。1923年，吉尔伯特·路易斯和梅尔·兰德尔提出另一种表述。

随着统计力学的发展，这一定律正如其他热力学定律一样，得到了解释，而不再是只能由实验验证的经验定律。

这一定律虽然由于适用条件的限制，应用范围并不如热力学第一、第二定律广泛，但仍有重要意义——特别是在物理化学领域。

The third law of thermodynamics is sometimes stated as follows, regarding the properties of systems in equilibrium at absolute zero temperature:

At absolute zero (zero kelvin), the system must be in a state with the minimum possible energy, and the above statement of the third law holds true provided that the perfect crystal has only one minimum energy state. Entropy is related to the number of accessible microstates, and for a system consisting of many particles, quantum mechanics indicates that there is only one unique state (called the ground state) with minimum energy. If the system does not have a well-defined order (if its order is glassy, for example), then in practice there will remain some finite entropy as the system is brought to very low temperatures as the system becomes locked into a configuration with non-minimal energy. The constant value is called the residual entropy of the system.