In this letter, a novel scheme for a wavelength-tunable pulse source (WTPS) is proposed and characterized. It is based on four-wave mixing (FWM) in a newly developed highly nonlinear fiber between a return-to-zero (RZ) pulsed signal at a fixed wavelength and a continuous wave probe tunable in wavelength. The corresponding FWM product acts as the WTPS, and is implemented in a 10-Gb/s, 160-km transmission experiment and in a 40-Gb/s multiplexing/demultiplexing experiment. The scheme can be expanded to a multiwavelength WTPS, which is demonstrated for two wavelengths. The introduced penalty using the WTPS compared to the original RZ pulses is negligible.