  We present the discovery and validation of a three-planet system orbiting the
nearby (31.1 pc) M2 dwarf star TOI-700 (TIC 150428135). TOI-700 lies in the
TESS continuous viewing zone in the Southern Ecliptic Hemisphere; observations
spanning 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 tests
enable us to rule out common astrophysical false-positive scenarios and
validate the system of planets. The outermost planet, TOI-700 d, has a radius
of $1.19\pm0.11$ R$_\oplus$ and resides in the conservative habitable zone of
its 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 host
Earth-sized planets in their habitable zones, TOI-700 exhibits low levels of
stellar activity, presenting a valuable opportunity to study potentially-rocky
planets over a wide range of conditions affecting atmospheric escape. While
atmospheric characterization of TOI-700 d with the James Webb Space Telescope
(JWST) will be challenging, the larger sub-Neptune, TOI-700 c (R = 2.63
R$_\oplus$), will be an excellent target for JWST and beyond. TESS is scheduled
to return to the Southern Hemisphere and observe TOI-700 for an additional 11
sectors in its extended mission, which should provide further constraints on
the known planet parameters and searches for additional planets and transit
timing variations in the system.
