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We present the discovery and validation of a three-planet system orbiting thenearby (31.1 pc) M2 dwarf star TOI-700 (TIC 150428135). TOI-700 lies in theTESS continuous viewing zone in the Southern Ecliptic Hemisphere; observationsspanning 11 sectors reveal three planets with radii ranging from 1 R$_\oplus$to 2.6 R$_\oplus$ and orbital periods ranging from 9.98 to 37.43 days.Ground-based follow-up combined with diagnostic vetting and validation testsenable us to rule out common astrophysical false-positive scenarios andvalidate the system of planets. The outermost planet, TOI-700 d, has a radiusof $1.19\pm0.11$ R$_\oplus$ and resides in the conservative habitable zone ofits host star, where it receives a flux from its star that is approximately 86%of the Earth's insolation. In contrast to some other low-mass stars that hostEarth-sized planets in their habitable zones, TOI-700 exhibits low levels ofstellar activity, presenting a valuable opportunity to study potentially-rockyplanets over a wide range of conditions affecting atmospheric escape. Whileatmospheric characterization of TOI-700 d with the James Webb Space Telescope(JWST) will be challenging, the larger sub-Neptune, TOI-700 c (R = 2.63R$_\oplus$), will be an excellent target for JWST and beyond. TESS is scheduledto return to the Southern Hemisphere and observe TOI-700 for an additional 11sectors in its extended mission, which should provide further constraints onthe known planet parameters and searches for additional planets and transittiming variations in the system.

The First Habitable Zone Earth-sized Planet from TESS. I: Validation of the TOI-700 System