Abstract: Microquasars share the same phenomenon of the relativistic jets to their cousins of quasars, showing redshifted and blueshifted emission lines in their spectra, if the jets are dominated by baryonic matter. Such relativistic jets are not expected from sources with supersoft X-ray spectra, based on previous observations and classical theory. We report the discovery of blueshifted broad Hα emission lines, with the projected velocities ~17 percent of the speed of light, in optical spectra of an ultraluminous supersoft X-ray source (ULS) in the nearby galaxy M81 (M81 ULS-1), which is the strong evidence of jets with relativistic speeds. The relativistic jets are not expected to be launched from white dwarfs, which is hard to reconcile with its constant soft X-rays, but the relativistic jets in a ULS may possibly be explained by a long speculated super-critically accreting black hole.