We present updated parameters for the star HD 209458 and its transiting giant planet. The stellar angular diameter θ = 0.2254 ± 0.0017 mas is obtained from the average ratio between the absolute flux observed with the Hubble Space Telescope and that of the best-fitting Kurucz model atmosphere. This angular diameter represents an improvement in precision of more than four times compared to available interferometric determinations. The stellar radius R⋆ = 1.20 ± 0.05 R⊙ is ascertained by combining the angular diameter with the Hipparcos trigonometric parallax, which is the main contributor to its uncertainty, and therefore the radius accuracy should be significantly improved with Gaia's measurements. The radius of the exoplanet Rp = 1.41 ± 0.06 RJ is derived from the corresponding transit depth in the light curve and our stellar radius. From the model fitting, we accurately determine the effective temperature, Teff = 6071 ± 20 K, which is in perfect agreement with the value of 6070 ± 24 K calculated from the angular diameter and the integrated spectral energy distribution. We also find precise values from recent Padova isochrones, such as R⋆ = 1.20 ± 0.06 R⊙ and Teff = 6099 ± 41 K. We arrive at a consistent picture from these methods and compare the results with those from the literature.