The authors report high-precision, high-cadence photometric measurements of the star HD 209458, which is known to have a planetary mass companion through radial velocity measurements. They detect two separate transit events that align with the radial velocity measurements. The detailed shape of the transit curve, influenced by the star's limb darkening and the planet's finite size, is clearly evident. Assuming stellar parameters of $1.1 R_{\odot}$ and $1.1 M_{\odot}$, the data are best interpreted as a gas giant with a radius of $1.27 \pm 0.02 R_{\text{Jup}}$ in an orbit with an inclination of $87.1 \pm 0.2^\circ$. The paper presents values for the planetary surface gravity, escape velocity, and average density, and discusses the numerous observations that are now warranted due to the discovery of a transiting planet. The findings confirm that the radial velocity variations in HD 209458 are indeed due to an orbiting planet, and provide valuable constraints on the planet's properties.The authors report high-precision, high-cadence photometric measurements of the star HD 209458, which is known to have a planetary mass companion through radial velocity measurements. They detect two separate transit events that align with the radial velocity measurements. The detailed shape of the transit curve, influenced by the star's limb darkening and the planet's finite size, is clearly evident. Assuming stellar parameters of $1.1 R_{\odot}$ and $1.1 M_{\odot}$, the data are best interpreted as a gas giant with a radius of $1.27 \pm 0.02 R_{\text{Jup}}$ in an orbit with an inclination of $87.1 \pm 0.2^\circ$. The paper presents values for the planetary surface gravity, escape velocity, and average density, and discusses the numerous observations that are now warranted due to the discovery of a transiting planet. The findings confirm that the radial velocity variations in HD 209458 are indeed due to an orbiting planet, and provide valuable constraints on the planet's properties.