SI unit of wavelength is meter.

The distance separating a wave's highest peak from its adjacent highest point, or its lowest point from its adjacent lowest point, is known as the wavelength. It is represented by the letter Lambda ($\lambda$). Since wavelength is defined as length in the definition provided, the S.I. unit of wavelength should be the same as the S.I. unit of length. Since we already know that a meter is the SI unit for length, a meter is also the SI unit for wavelength.

The distance between two crests or two troughs is called as wavelength in a wave. As wavelength is measured in distance, its SI unit is meters.