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We present the discovery of TOI-197.01, the first transiting planetidentified by the Transiting Exoplanet Survey Satellite (TESS) for whichasteroseismology of the host star is possible. TOI-197 (HIP116158) is a bright(V=8.2 mag), spectroscopically classified subgiant which oscillates with anaverage frequency of about 430 muHz and displays a clear signature of mixedmodes. The oscillation amplitude confirms that the redder TESS bandpasscompared to Kepler has a small effect on the oscillations, supporting theexpected yield of thousands of solar-like oscillators with TESS 2-minutecadence observations. Asteroseismic modeling yields a robust determination ofthe host star radius (2.943+/-0.064 Rsun), mass (1.212 +/- 0.074 Msun) and age(4.9+/-1.1 Gyr), and demonstrates that it has just started ascending thered-giant branch. Combining asteroseismology with transit modeling andradial-velocity observations, we show that the planet is a "hot Saturn"(9.17+/-0.33 Rearth) with an orbital period of ~14.3 days, irradiance of343+/-24 Fearth, moderate mass (60.5 +/- 5.7 Mearth) and density (0.431+/-0.062gcc). The properties of TOI-197.01 show that the host-star metallicity - planetmass correlation found in sub-Saturns (4-8 Rearth) does not extend to largerradii, indicating that planets in the transition between sub-Saturns andJupiters follow a relatively narrow range of densities. With a density measuredto ~15%, TOI-197.01 is one of the best characterized Saturn-sized planets todate, augmenting the small number of known transiting planets around evolvedstars and demonstrating the power of TESS to characterize exoplanets and theirhost stars using asteroseismology.

A Hot Saturn Orbiting An Oscillating Late Subgiant Discovered by TESS