  We report the discovery of GJ 1252 b, a planet with a radius of 1.193 $\pm$
0.074 $R_{\oplus}$ and an orbital period of 0.52 days around an M3-type star
(0.381 $\pm$ 0.019 $M_{\odot}$, 0.391 $\pm$ 0.020 $R_{\odot}$) located 20.385
$\pm$ 0.019 pc away. We use TESS data, ground-based photometry and
spectroscopy, Gaia astrometry, and high angular resolution imaging to show that
the transit signal seen in the TESS data must originate from a transiting
planet. We do so by ruling out all false positive scenarios that attempt to
explain the transit signal as originating from an eclipsing stellar binary.
Precise Doppler monitoring also leads to a tentative mass measurement of 2.09
$\pm$ 0.56 $M_{\oplus}$. The host star proximity, brightness ($V$ = 12.19 mag,
$K$ = 7.92 mag), low stellar activity, and the system's short orbital period
make this planet an attractive target for detailed characterization, including
precise mass measurement, looking for other objects in the system, and planet
atmosphere characterization.
