  We report the discovery of an intermediate-mass transiting brown dwarf,
TOI-503b, from the TESS mission. TOI-503b is the first brown dwarf discovered
by TESS and orbits a metallic-line A-type star with a period of $P=3.6772 \pm
0.0001$ days. The light curve from TESS indicates that TOI-503b transits its
host star in a grazing manner, which limits the precision with which we measure
the brown dwarf's radius ($R_b = 1.34^{+0.26}_{-0.15} R_J$). We obtained
high-resolution spectroscopic observations with the FIES, Ond\v{r}ejov, PARAS,
Tautenburg, and TRES spectrographs and measured the mass of TOI-503b to be $M_b
= 53.7 \pm 1.2 M_J$. The host star has a mass of $M_\star = 1.80 \pm 0.06
M_\odot$, a radius of $R_\star = 1.70 \pm 0.05 R_\odot$, an effective
temperature of $T_{\rm eff} = 7650 \pm 160$K, and a relatively high metallicity
of $0.61\pm 0.07$ dex. We used stellar isochrones to derive the age of the
system to be $\sim$180 Myr, which places its age between that of RIK 72b (a
$\sim$10 Myr old brown dwarf in the Upper Scorpius stellar association) and AD
3116b (a $\sim$600 Myr old brown dwarf in the Praesepe cluster). We argue that
this brown dwarf formed in-situ, based on the young age of the system and the
long circularization timescale for this brown dwarf around its host star.
TOI-503b joins a growing number of known short-period, intermediate-mass brown
dwarfs orbiting main sequence stars, and is the second such brown dwarf known
to transit an A star, after HATS-70b. With the growth in the population in this
regime, the driest region in the brown dwarf desert ($35-55 M_J \sin{i}$) is
reforesting and its mass range shrinking.
