反质子（英语：antiproton）是质子的反粒子，其质量及自旋与质子相同，但电荷及磁矩则与质子相反，带有与电子相同的负电荷。

保罗·狄拉克在他的1933年诺贝尔物理学奖演讲中预言反质子的存在。1955年，加州大学柏克莱分校物理学家埃米利奥·塞格雷和欧文·张伯伦透过粒子加速器，而发现了这种反粒子，他们二人于1959年获得诺贝尔物理学奖。

反质子是质子的反粒子，符号为p，其质量、自旋与质子相同，且寿命也与质子相当；但其电荷及磁矩则与质子相反，且带有与电子同电量的负电荷。这些性质与量子场理论的基础－－CPT对称理论预测相符合。一个反质子是由两个上反夸克及一个下反夸克所组成（u u d ）。虽然反质子本身是稳定的，但由于反质子与质子撞击会发生湮灭的现象，并且转化为能量，是故反粒子无法在一般的自然环境中保存。

The antiproton, p, (pronounced p-bar) is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived since any collision with a proton will cause both particles to be annihilated in a burst of energy.

The existence of the antiproton with −1 electric charge, opposite to the +1 electric charge of the proton, was predicted by Paul Dirac in his 1933 Nobel Prize lecture. Dirac received the Nobel Prize for his previous 1928 publication of his Dirac Equation that predicted the existence of positive and negative solutions to the Energy Equation () of Einstein and the existence of the positron, the antimatter analog to the electron, with positive charge and opposite spin.