  We report discovery of a bright, nearby ($G = 13.8;\,\,d = 480\,\rm pc$)
Sun-like star orbiting a dark object. We identified the system as a black hole
candidate via its astrometric orbital solution from the Gaia mission. Radial
velocities validated and refined the Gaia solution, and spectroscopy ruled out
significant light contributions from another star. Joint modeling of radial
velocities and astrometry constrains the companion mass to $M_2 = 9.62\pm
0.18\,M_{\odot}$. The spectroscopic orbit alone sets a minimum companion mass
of $M_2>5\,M_{\odot}$; if the companion were a $5\,M_{\odot}$ star, it would be
$500$ times more luminous than the entire system. These constraints are
insensitive to the mass of the luminous star, which appears as a
slowly-rotating G dwarf ($T_{\rm eff}=5850\,\rm K$, $\log g = 4.5$,
$M=0.93\,M_{\odot}$), with near-solar metallicity ($\rm [Fe/H] = -0.2$) and an
unremarkable abundance pattern. We find no plausible astrophysical scenario
that can explain the orbit and does not involve a black hole. The orbital
period, $P_{\rm orb}=185.6$ days, is longer than that of any known stellar-mass
black hole binary. The system's modest eccentricity ($e=0.45$), high
metallicity, and thin-disk Galactic orbit suggest that it was born in the Milky
Way disk with at most a weak natal kick. How the system formed is uncertain.
Common envelope evolution can only produce the system's wide orbit under
extreme and likely unphysical assumptions. Formation models involving triples
or dynamical assembly in an open cluster may be more promising. This is the
nearest known black hole by a factor of 3, and its discovery suggests the
existence of a sizable population of dormant black holes in binaries. Future
Gaia releases will likely facilitate the discovery of dozens more.
