巨正则系综（英语：grand canonical ensemble）是正则系综的推广。是统计力学系综的一种。每个系综内的体系不仅可以和其他体系交换能量，也可以交换粒子，但系综内各体系的能量总和以及粒子数总和都是固定的。当然系综内的体系总数也是固定不变的。而且各体系的体积是保持在一个固定值上。这个系综对应于具有恒定温度和化学势的体系。

巨正则系综由两个参数决定，（是玻尔兹曼常数，是温度）和逸度（或等价地，化学势）。其中决定能量交换；逸速度决定粒子交换。其配分函数为：

配分函数的对数可以和压强联系起来。

In statistical mechanics, a grand canonical ensemble is the statistical ensemble that is used to represent the possible states of a mechanical system of particles that is being maintained in thermodynamic equilibrium (thermal and chemical) with a reservoir. The system is said to be open in the sense that the system can exchange energy and particles with a reservoir, so that various possible states of the system can differ in both their total energy and total number of particles. The system's volume, shape, and other external coordinates are kept the same in all possible states of the system.