  Stellar-mass black holes are the final remnants of stars born with more than
15 solar masses. Billions are expected to reside in the Local Group, yet only
few are known, mostly detected through X-rays emitted as they accrete material
from a companion star. Here, we report on VFTS 243: a massive X-ray faint
binary in the Large Magellanic Cloud. With an orbital period of 10.4-d, it
comprises an O-type star of 25 solar masses and an unseen companion of at least
nine solar masses. Our spectral analysis excludes a non-degenerate companion at
a 5-sigma confidence level. The minimum companion mass implies that it is a
black hole. No other X-ray quiet black hole is unambiguously known outside our
Galaxy. The (near-)circular orbit and kinematics of VFTS 243 imply that the
collapse of the progenitor into a black hole was associated with little or no
ejected material or black-hole kick. Identifying such unique binaries
substantially impacts the predicted rates of gravitational-wave detections and
properties of core-collapse supernovae across the Cosmos.
