We report the discovery of the closest known black hole candidate as a binary companion to V723 Mon. V723 Mon is a nearby ($dsim460,rm pc$), bright ($Vsimeq8.3$~mag), evolved ($T_{rm eff, giant}simeq4440$~K, and $L_{rm giant}simeq173~L_odot$) red giant in a high mass function, $f(M)=1.72pm 0.01~M_odot$, nearly circular binary ($P=59.9$ d, $esimeq 0$). 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 $qsimeq0.33pm0.02$, a companion mass of $M_{rm comp}=3.04pm0.06~M_odot$, a stellar radius of $R_{rm giant}=24.9pm0.7~R_odot$, and a giant mass of $M_{rm giant}=1.00pm0.07~ M_odot$. 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 $L_{rm X}simeq7.6times10^{29}~rm ergs~s^{-1}$, 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.
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