  We report the discovery of the closest known black hole candidate as a binary
companion to V723 Mon. V723 Mon is a nearby ($d\sim460\,\rm pc$), bright
($V\simeq8.3$~mag), evolved ($T_{\rm eff, giant}\simeq4440$~K, and $L_{\rm
giant}\simeq173~L_\odot$) red giant in a high mass function, $f(M)=1.72\pm
0.01~M_\odot$, nearly circular binary ($P=59.9$ d, $e\simeq 0$). V723 Mon is a
known variable star, previously classified as an eclipsing binary, but its
All-Sky Automated Survey (ASAS), Kilodegree Extremely Little Telescope (KELT),
and Transiting Exoplanet Survey Satellite (TESS) light curves are those of a
nearly edge-on ellipsoidal variable. Detailed models of the light curves
constrained by the period, radial velocities and stellar temperature give an
inclination of $87.0^\circ{}^{+1.7^{\circ}}_{-1.4^{\circ}} $, a mass ratio of
$q\simeq0.33\pm0.02$, a companion mass of $M_{\rm comp}=3.04\pm0.06~M_\odot$, a
stellar radius of $R_{\rm giant}=24.9\pm0.7~R_\odot$, and a giant mass of
$M_{\rm giant}=1.00\pm0.07~ M_\odot$. We identify a likely non-stellar, diffuse
veiling component with contributions in the $B$ and $V$-band of ${\sim}63\%$
and ${\sim}24\%$, respectively. The SED and the absence of continuum eclipses
imply that the companion mass must be dominated by a compact object. We do
observe eclipses of the Balmer lines when the dark companion passes behind the
giant, but their velocity spreads are low compared to observed accretion disks.
The X-ray luminosity of the system is $L_{\rm X}\simeq7.6\times10^{29}~\rm
ergs~s^{-1}$, corresponding to $L/L_{\rm edd}{\sim}10^{-9}$. The simplest
explanation for the massive companion is a single compact object, most likely a
black hole in the "mass gap".
