Measurements from the Retarding Ion Mass Spectrometer (RIMS) on Dynamics Explorer (DE) have, for the first time, revealed a supersonic polar wind (Mach number is greater than 1) along polar cap field lines. The observations reported were obtained on the nightside (22:30 to 23:30 MLT) from 65¿ to 81¿ invariant latitude and at altitudes near 2 RE. Fitting the data using a thin-sheath model gives a range of temperatures of 0.1 to 0.2 eV with corresponding flow velocities of 25 to 16 km s-1 over the estimated range of spacecraft potential of +3 to +5 V. For these values the Mach number ranged from 5.1 to 2.6 (with a most likely value of 3). Characteristics of the H+ flow are in general agreement with those predicted by ''classical'' polar wind theory, but high variability of the He+/H+ ratio was observed.