We report the experimental observation of ac-driven phase-locked motion of a topological soliton at a nonzero average velocity in a periodically modulated lossy medium. The velocity is related by a resonant condition to the driving frequency. The observation is made in terms of the current-voltage, I(V), characteristics for a fluxon trapped in an annular Josephson junction placed into dc magnetic field. Large zero-crossing constant-voltage steps, exactly corresponding to the resonantly locked soliton motion at different orders of the resonance, are found on the experimental I(V) curves. A measured dependence of the size of the steps versus the external magnetic field is in good agreement with predictions of an analytical model based on the balance equation for the fluxon’s energy. The effect has a potential application as a low-frequency voltage standard.