在理论物理学里，贝尔定理（Bell's theorem）表明

贝尔定理是一种不可行定理，又知名为贝尔不等式。这定理在物理学和科学哲学里异常重要，因为这定理意味着量子物理必需违背定域性原理或反事实确定性 。发表于1964年，贝尔定理是因爱尔兰物理学家约翰·贝尔而命名。

贝尔定理的实验验证所得到的结果，符合量子力学理论的预测，并且显示某些量子效应貌似能够以超光速行进。由于这验证结果，所有归类为隐变量理论、经得起考验的量子理论都只能限制为非定域种类。请特别注意，所有至今完成的贝尔定理的实验验证，没有一个实验能够完全满足贝尔定理所有内含的要求。于此，没有任何结果能够给出决定性的总论。

Bell's theorem is a ‘no-go theorem’ that draws an important distinction between quantum mechanics (QM) and the world as described by classical mechanics. This theorem is named after John Stewart Bell.

In its simplest form, Bell's theorem states:

Cornell solid-state physicist David Mermin has described the appraisals of the importance of Bell's theorem in the physics community as ranging from "indifference" to "wild extravagance". Lawrence Berkeley particle physicist Henry Stapp declared: "Bell's theorem is the most profound discovery of science."