Stellar theory predicts a forbidden range of black-hole masses between approximately 50 M⊙ and 130 M⊙ owing to pair-instability supernovae1–7, but evidence for such a gap in the mass distribution from gravitational-wave astronomy has proved elusive. Early hints of a cut-off in black-hole masses at about 45 M⊙ disappeared with the subsequent discovery of more massive binary black holes8,9. Here we report evidence of the pair-instability gap i