We demonstrate a simple and reliable scheme for wavelength switching in an actively harmonic mode-locked Fabry–Perot laser diode (FPLD) that is coupled to a highly dispersive external fiber cavity. Wavelength switching between FPLD modes is achieved by detuning of the modulation frequency applied to the FPLD. A side-mode suppression ratio of as much as 40\text{\hspace{0.17em} dB} is maintained, and the supermode noise suppression ratio is more than 50\text{\hspace{0.17em} dB} for the whole wavelength switching operation range of 4.7\text{\hspace{0.17em} nm} when the pulse repetition rate is \sim 2\text{\hspace{0.17em} GHz} . The wavelength- switching mechanism of our laser is based on the wavelength-dependent cavity resonance frequency shift that is due to chromatic dispersion, and we verify that our experimental results closely coincide with analytical results.