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We report the discovery, mass, and radius determination of TOI-1801 b, atemperate mini-Neptune around a young M dwarf. TOI-1801 b was observed in TESSsectors 22 and 49, and the alert that this was a TESS planet candidate with aperiod of 21.3 days went out in April 2020. However, ground-based follow-upobservations, including seeing-limited photometry in and outside transittogether with precise radial velocity (RV) measurements with CARMENES and HIRESrevealed that the true period of the planet is 10.6 days. These observationsalso allowed us to retrieve a mass of 5.74 $\pm$ 1.46 $M_\oplus$, whichtogether with a radius of 2.08 $\pm$ 0.12 $R_\oplus$, means that TOI-1801 b ismost probably composed of water and rock, with an upper limit of 2\% by mass ofH$_{2}$ in its atmosphere. The stellar rotation period of 16 days is readilydetectable in our RV time series and in the ground-based photometry. We deriveda likely age of 600--800 Myr for the parent star TOI-1801, which means thatTOI-1801 b is the least massive young mini-Neptune with precise mass and radiusdeterminations. Our results suggest that if TOI-1801 b had a larger atmospherein the past, it must have been removed by some evolutionary mechanism ontimescales shorter than 1 Gyr.

TOI-1801 b: A temperate mini-Neptune around a young M0.5 dwarf