Abstract The imminent launch of space telescopes designed to probe the atmospheres of exoplanets has prompted new efforts to prioritize the thousands of transiting planet candidates for follow-up characterization. We report the detection and confirmation of TOI-1842b, a warm Saturn identified by TESS and confirmed with ground-based observations from M inerva -Australis, NRES, and the Las Cumbres Observatory Global Telescope. This planet has a radius of 1.04 − 0.05 + 0.06 R J , a mass of 0.214 − 0.038 + 0.040 M J , an orbital period of 9.5739 − 0.0001 + 0.0002 days, and an extremely low density ( ρ = 0.252 ± 0.091 g cm −3 ). TOI-1842b has among the best known combinations of large atmospheric scale height (893 km) and host-star brightness ( J = 8.747 mag), making it an attractive target for atmospheric characterization. As the host star is beginning to evolve off the main sequence, TOI-1842b presents an excellent opportunity to test models of gas giant reinflation. The primary transit duration of only 4.3 hr also makes TOI-1842b an easily-schedulable target for further ground-based atmospheric characterization.