Observations of quasars reveal that many supermassive black holes (BHs) were in place less than 700 Myr after the Big Bang. However, the origin of the first BHs remains a mystery. Seeds of the first BHs are postulated to be either light (that is, 10−100 M⊙), remnants of the first stars, or heavy (that is, 10−105 M⊙), originating from the direct collapse of gas clouds. Here, harnessing recent data from the Chandra X-ray Observatory, we report the detection of an X-ray-luminous massive BH in a gravitationally lensed galaxy identified by the James Webb Space Telescope at redshift z ≈ 10.3 behind the cluster lens Abell 2744. This heavily obscured quasar with a bolometric luminosity of ~5 × 1045 erg s−1 harbours an ~107−108 M⊙ BH assuming accretion at the Eddington limit. This mass is comparable to the inferred stellar mass of its host galaxy, in contrast to what is found in the local Universe wherein the BH mass is ~0.1% of the host galaxy’s stellar mass. The combination of such a high BH mass and large BH-to-galaxy stellar mass ratio just ~500 Myr after the Big Bang was theoretically predicted and is consistent with a picture wherein BHs originated from heavy seeds. A lensed quasar at redshift z ≈ 10.3, seen in X-rays, hosts a supermassive black hole of mass similar to that of its host galaxy. The large black-hole mass at a young age, as well as the amount of X-rays it produces, suggest that the black hole formed from the collapse of a huge cloud of gas.