波的相速度或相位速度(phase velocity)，或简称相速，是指波的相位在空间中传递的速度，换句话说，波的任一频率成分所具有的相位即以此速度传递。可以挑选波的任一特定相位来观察(例如波峰)，则此处会以相速度前行。相速度可借由波的频率f与波长λ，或者是角频率ω与波数(wave number) k的关系式表示：

注意到波的相速度不必然与波的群速度相同，相速是波包中某一单频波的相位移动速度；群速度代表的是「振幅变化」(或说波包)的传递速度，表示一段波包的包络面上具有某特性（如幅值最大或最小）的点的传播速度。

The phase velocity of a wave is the rate at which the phase of the wave propagates in space. This is the velocity at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave (for example, the crest) will appear to travel at the phase velocity. The phase velocity is given in terms of the wavelength λ (lambda) and period T as