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We present $Spitzer$ 4.5$\mu$m observations of the transit of TOI-700 d, ahabitable zone Earth-sized planet in a multiplanet system transiting a nearbyM-dwarf star (TIC 150428135, 2MASS J06282325-6534456). TOI-700 d has a radiusof $1.144^{+0.062}_{-0.061}R_\oplus$ and orbits within its host star'sconservative habitable zone with a period of 37.42 days ($T_\mathrm{eq} \sim269$K). TOI-700 also hosts two small inner planets(R$_b$=$1.037^{+0.065}_{-0.064}R_\oplus$ &R$_c$=$2.65^{+0.16}_{-0.15}R_\oplus$) with periods of 9.98 and 16.05 days,respectively. Our $Spitzer$ observations confirm the TESS detection of TOI-700d and remove any remaining doubt that it is a genuine planet. We analyze the$Spitzer$ light curve combined with the 11 sectors of TESS observations and atransit of TOI-700 c from the LCOGT network to determine the full systemparameters. Although studying the atmosphere of TOI-700 d is not likelyfeasible with upcoming facilities, it may be possible to measure the mass ofTOI-700 d using state-of-the-art radial velocity instruments (expected RVsemi-amplitude of $\sim$70 cm/s).

The First Habitable Zone Earth-Sized Planet From TESS II: $Spitzer$ Confirms TOI-700 d