在量子力学里，微观粒子有时会显示出波动性（这时粒子性较不显着），有时又会显示出粒子性（这时波动性较不显着），在不同条件下分别表现出波动或粒子的性质。这种量子行为称为波粒二象性（英语：wave-particle duality），是微观粒子的基本属性之一。

波粒二象性指的是微观粒子显示出的波动性与粒子性。这是量子力学的基要概念，是专门针对古典概念无法完整描述量子物体的物理行为而提出的假说。标准的量子力学诠释将这佯谬解释为宇宙的基础性质，而其它种诠释可能会有标新立异的论述。本条目主要采用的是学术界广泛认可的哥本哈根诠释来解释量子行为。采用这种诠释，波粒二象性是更广义的互补性概念的一方面，即量子现象可以用一种方法或另外一种共轭方法来观察，但不能同时用两种相互共轭的方法来观察。

Wave–particle duality is the concept that every elementary particle or quantic entity may be partly described in terms not only of particles, but also of waves. It expresses the inability of the classical concepts "particle" or "wave" to fully describe the behavior of quantum-scale objects. As Einstein wrote: "It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do".