物理学中，汤姆孙散射是指电磁辐射和一个自由带电粒子产生的弹性散射。入射电磁波的电场使粒子加速，从而激发粒子产生和入射波频率相同的辐射（散射波）。汤姆孙散射是康普顿散射在低能量区的近似。汤姆孙散射是等离子物理学中的一个重要现象，它首先由英国物理学家约瑟夫·汤姆孙解释。

只要粒子的运动是非相对论性的（即速度远小于光速），粒子加速的主要原因都来自入射波的电场分量，而磁场的作用可被忽略。粒子将会在电场振动的方向上开始运动，从而产生电磁偶极辐射。运动粒子在垂直于运动方向上的辐射最强，而辐射沿着粒子的运动方向产生偏振。从而，取决于观察者的位置，从一个小体元散射出的电磁波存在程度不同的偏振。

Thomson scattering is the elastic scattering of electromagnetic radiation by a free charged particle, as described by classical electromagnetism. It is just the low-energy limit of Compton scattering: the particle kinetic energy and photon frequency are the same before and after the scattering. This limit is valid as long as the photon energy is much less than the mass energy of the particle: , or equivalently, if the wavelength of the light is much greater than the Compton wavelength of the particle. The classical description is also the low-intensity limit.