Starburst wind models predict that metals and energy are primarily carried out of the disk by hot gas (T > 10(6) K), but the low energy resolution of X-ray CCD observations results in large uncertainties on the mass and energy loading. Here, we present evidence for a fast soft X-ray wind from the prototypical starburst galaxy M82 using deep archival observations from the Reflection Grating Spectrometer on XMM-Newton. After characterizing the complex line-spread function for the spatially extended outflow (approximate to 4 '), we perform emission-line fitting to measure the velocity dispersion, sigma(v), from O viii (0.65, 0.77 keV), Ne x (1.02 keV), and Mg xii (1.47 keV). For the T approximate to 3 x 10(6) K gas, O viii yields a velocity dispersion of sigma(v)=1160(-90)(+100) km s(-1), implying a wind speed that is significantly above the escape velocity (v(esc) less than or similar to 450 km s(-1)). Ne X (sigma(v)=550(-150)(+130) km s(-1)) and Mg XII (sigma(v) < 370 km s(-1)) show less velocity broadening than O viii, hinting at a lower wind speed or smaller opening angle on the more compact spatial scales traced by the T approximate to (0.6-1) x 10(7) K gas. Alternatively, these higher energy emission lines may be dominated by shock-heated gas in the interstellar medium. Future synthesis of these measurements with performance verification observations of the E = 2-12 keV wind in M82 from the Resolve microcalorimeter on the X-ray Imaging and Spectroscopy Mission will inform the phase structure and energy budget of the hot starburst wind.
