We present and discuss the UV spectrum of the supergiant star in NGC206-277 in M 31, obtained with the Faint Object Spectrograph (FOS) on Hubble Space Telescope (HST). An optical blue spectrum taken at Kitt Peak National Observatory (KPNO) is also shown. From the optical and UV line spectra we classify the star B1.5Ia. We fit the continuum far-UV flux distribution, deriving Teff = 20,000 K, and with this temperature we find log g = 2.5 by comparing the Hgamma line and He I lines with predictions from Non-Local Thermodynamic Equilibrium (NLTE) models. We analyze the stellar UV wind line profiles with the SEI method and derive a terminal velocity of upsiloninfinity approximately equal to 700 km/s. We fit the Halpha emission line obtained at the WHT with a newly developed (by H.J.G.L.M.L.) code similar to the SEI method for UV lines, and derive dm/dt = 1.3 +/- 0.5 10-6 solar mass/yr. We find that the P Cygni profiles of the M 31 star have slightly weaker and narrower absorptions, and no emission component. We correct our previously published mass-loss estimate for an O star (NGC206-231) in the same association. The correct value is dm/dt approximately equals 10-6 solar mass/yr, assuming solar abundances.