We observed saturated absorption spectra of potassium at 404.8 nm with a frequency-stabilized violet diode laser. A diffraction grating in the Littrow configuration was used to get a single-mode output, and simultaneous resonant optical feedback from a confocal Fabry-Perot cavity was employed to stabilize the laser frequency. The observed short-term frequency fluctuations are 300 kHz for 1 s, and the upper limit on the instantaneous linewidth is 1.5 MHz. The optically locked laser can be continuously scanned over 4 GHz. The observed typical linewidth of the potassium transition is 3.6 MHz with a signal-to-noise ratio of 17. The transition is suitable for stabilizing frequency of violet diode lasers to a sub-MHz level.