The exoplanet HD 118203 b, orbiting a bright (V = 8.05) host star, was discovered using the radial velocity method by da Silva et al. (2006), but was not previously known to transit. TESS photometry has revealed that this planet transits its host star. Five planetary transits were observed by TESS, allowing us to measure the radius of the planet to be $1.133 pm 0.031 R_J$, and to calculate the planet mass to be $2.173 pm 0.078 M_J$. The host star is slightly evolved with an effective temperature of $T_{rm eff} = 5692 pm 83$ K and a surface gravity of ${rm log}(g) = 3.891 pm 0.019$. With an orbital period of $6.134980 pm 0.000038$ days and an eccentricity of $0.316 pm 0.021$, the planet occupies a transitional regime between circularized hot Jupiters and more dynamically active planets at longer orbital periods. The host star is among the ten brightest known to have transiting giant planets, providing opportunities for both planetary atmospheric and asteroseismic studies.
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