在粒子物理学中，π介子是以下三种次原子粒子之一：π、π和π。π介子是最重要的介子之一，在揭示强核力的低能量特性中起着重要的作用。

π介子拥有0自旋，由第一代夸克组成。在夸克模型中，一个上夸克和一个反下夸克构成一个π，一个下夸克和一个反上夸克构成一个π，它们互为反粒子。中性的组合——上夸克和反上夸克、下夸克和反下夸克组成π，它们拥有相同的量子数，因而只能在叠加中出现。最低能量的叠加是π，它的反粒子就是自己。

π介子拥有139.6MeV/c的质量，和2.6×10s的平均寿命。它们因弱作用而衰变。主要的衰变形式（占99.9877%）是纯轻子型衰变，变成一个μ子和一个中微子。

In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: π) is any of three subatomic particles: π0, π+, and π−. Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons (and, more generally, the lightest hadrons), because they are composed of the lightest quarks (the u and d quarks). They are unstable, with the charged pions π+ and π− decaying with a mean lifetime of 26 nanoseconds (6992260000000000000♠2.6×10 seconds), and the neutral pion π0 decaying with a much shorter lifetime of 6983840000000000000♠8.4×10 seconds. Charged pions most often decay into muons and muon neutrinos, while neutral pions generally decay into gamma rays.