We report the discovery of a planet in the microlensing event OGLE-2018-BLG-1269 with a planet-host mass ratio q~6x10^-4^, i.e., 0.6 times smaller than the Jupiter/Sun mass ratio. Combined with the Gaia parallax and proper motion, a strong one-dimensional constraint on the microlens parallax vector allows us to significantly reduce the uncertainties of lens physical parameters. A Bayesian analysis that ignores any information about light from the host yields that the planet is a cold giant (M_2_=0.69_-0.22_^+0.44^M_J_) orbiting a Sun-like star (M_1_=1.13_-0.35_^+0.72^M_{sun}_) at a distance of D_L_=2.56_-0.62_^+0.92^kpc. The projected planet-host separation is a_{perp}_=4.61_-1.17_^+1.70^au. Using Gaia astrometry, we show that the blended light lies <~12mas from the host and therefore must be either the host star or a stellar companion to the host. An isochrone analysis favors the former possibility at >99.6%. The host is therefore a subgiant. For host metallicities in the range of 0.0<=[Fe/H]<=+0.3, the host and planet masses are then in the range of 1.16<=M_1_/M_{sun}_<=1.38 and 0.74<=M_2_/M_J_<=0.89, respectively. Low host metallicities are excluded. The brightness and proximity of the lens make the event a strong candidate for spectroscopic follow-up both to test the microlensing solution and to further characterize the system.