We report a spectroscopic orbit with period P = 3.52433 ± 0.00027 days for the planetary companion that transits the solar-type star HD 209458. For the metallicity, mass, and radius of the star, we derive [Fe/H] = 0.00 ± 0.02, M* = 1.1 ± 0.1 M☉, and R* = 1.2 ± 0.1 R☉. This is based on a new analysis of the iron lines in our HIRES template spectrum and also on the absolute magnitude, effective temperature, and color of the star, and it uses isochrones from four different sets of stellar evolution models. Using these values for the stellar parameters, we reanalyze the transit data and derive an orbital inclination of i = 86fdg1 ± 1fdg6. For the planet, we derive a mass of Mp = 0.69 ± 0.05 MJup, a radius of Rp = 1.40 ± 0.17 RJup, and a density of ρ = 0.31 ± 0.07 g cm⁻³.