GZK极限，是以提出者Greisen、Zatsepin、Kuzmin三人姓氏之首字母为名的理论上限，描述源自远处的宇宙射线应有的理论上限值。

这项极限是在1966年由Kenneth Greisen、Vadim Kuzmin与Georgiy Zatsepin三人所计算，其基础为宇宙微波背景辐射（cosmic microwave background radiation, CMB radiation）与宇宙射线的预期交互作用。预测中指出宇宙射线所带的能量如果超过阈值5×10 电子伏特 (eV)则会与宇宙微波背景的光子发生交互作用，产生Π介子（pion）。这样的作用会持续发生，一直到射线粒子的能量低于Π介子产生阈值。因为此交互作用相关的平均自由程其值甚低，举例来说，起源处距离地球远大于50 百万秒差距 (Mpc)的星系外宇宙射线（extragalactic cosmic rays）若其能量大于此阈值者，则不可能在地球上观测到；而此距离内又不存在目前已知可以产生此般能量的宇宙射线源。

The Greisen–Zatsepin–Kuzmin limit (GZK limit) is a theoretical upper limit on the energy of cosmic rays (high energy charged particles from space) coming from "distant" sources. The limit is 5×10 eV, or about 8 joules. The limit is set by slowing-interactions of cosmic ray protons with the microwave background radiation over long distances (~160 million light-years). The limit is at the same order of magnitude as the upper limit for energy at which cosmic rays have experimentally been detected. For example, one extreme-energy cosmic ray has been detected which appeared to possess a record 3.12×10 eV (50 joules) of energy (about the same as the kinetic energy of a 60 mph baseball).