We report measurement of the trigonometric parallax of W51 Main/South using the Very Long Baseline Array. We measure a value of 0.185+/-0.010mas corresponding to a distance of 5.41^+0.31^-0.28_kpc. W51 Main/South is a well-known massive star-forming region near the tangent point of the Sagittarius spiral arm of the Milky Way. Our distance to W51 yields an estimate of the distance to the Galactic center of R_0=8.3+/-0.46(statistical)+/-1.0(systematic)kpc by simple geometry. Combining the parallax and proper motion measurements for W51, we obtained the full-space motion of this massive star-forming region. We find W51 is in a nearly circular orbit about the Galactic center. The H_2_O masers used for our parallax measurements trace four powerful bipolar outflows within a 0.4pc size region, some of which are associated with dusty molecular hot cores and/or hyper- or ultra-compact HII regions.