散射矩阵,又称S矩阵，是物理学中描述散射过程的一个主要观测量。

现代高能物理的发展，同其他物理学一样是理论和实验的互动，而这种互动主要的桥梁就是散射矩阵。

假设散射源为很好的定域散射源，与被散射粒子的相互作用局限在有限的空间范围内，那幺，无穷远时间以前粒子处于一个自由态，称为入态，记为|Ψ>_in；无穷远时间之后粒子也是处于一个自由态，称为出态，记为|Ψ>_out。 入态到初态，相互作用可以用一个矩阵描述，记为S，那幺就有：

这就是散射矩阵的定义。

散射矩阵直接与可观测的物理量相联系，但是我们在量子场论中处理的是场，两者如何联系？或者说如何从量子场论计算散射矩阵？我们还要利用一个LSZ约化规则，它联系了量子场论中的格林函数和可观测的散射矩阵。这使得理论能够预言实验。

In physics, the S-matrix or scattering matrix relates the initial state and the final state of a physical system undergoing a scattering process. It is used in quantum mechanics, scattering theory and quantum field theory.

More formally, the S-matrix is defined as the unitary matrix connecting asymptotic particle states in the Hilbert space of physical states (scattering channels). While the S-matrix may be defined for any background (spacetime) that is asymptotically solvable and has no event horizons, it has a simple form in the case of the Minkowski space. In this special case, the Hilbert space is a space of irreducible unitary representations of the inhomogeneous Lorentz group (the Poincaré group); the S-matrix is the evolution operator between time equal to minus infinity (the distant past), and time equal to plus infinity (the distant future). It is defined only in the limit of zero energy density (or infinite particle separation distance).