We report the gravitational microlensing discovery of a sub-Saturn mass planet, MOA-2009-BLG-319Lb, orbiting a K- or M-dwarf star in the inner Galactic disk or Galactic bulge. The high-cadence observations of the MOA-II survey discovered this microlensing event and enabled its identification as a high-magnification event approximately 24 hr prior to peak magnification. As a result, the planetary signal at the peak of this light curve was observed by 20 different telescopes, which is the largest number of telescopes to contribute to a planetary discovery to date. The microlensing model for this event indicates a planet-star mass ratio of q = (3.95 ± 0.02) × 10^(–4) and a separation of d = 0.97537 ± 0.00007 in units of the Einstein radius. A Bayesian analysis based on the measured Einstein radius crossing time, t_E, and angular Einstein radius, θ_E, along with a standard Galactic model indicates a host star mass of M_L = 0.38^(+0.34)_(–0.18) M_☉ and a planet mass of M_p = 50^(+44)_(–24) M_⊕, which is half the mass of Saturn. This analysis also yields a planet-star three-dimensional separation of a = 2.4^(+1.2)_(–0.6) AU and a distance to the planetary system of D_L = 6.1^(+1.1)_(–1.2) kpc. This separation is ~2 times the distance of the snow line, a separation similar to most of the other planets discovered by microlensing
