Abstract Optical interfaces that connect long-lived spin qubits to photons are a central requirement for quantum networking and distributed quantum information processing. At present, solid-state atomic defects are leading candidates because of their inherent spin and optical coherence 1,2 . Building on these advancements, synthetically tailored molecular systems represent a fundamental change in the field, using precise atomic control and consistent bottom-up assembly 3,4 . However, a molecular platform that concurrently satisfies the demands of a robust spin–photon interface—namely, the combination of bright fluorescence, high spectral stability and long ground-state spin lifetimes—has yet to be realized at the single-molecule level. Here we show that a triplet ground-state carbene molecule, embedded within a structurally matched host crystal, functions as a robust spin–photon interface with single-molecule addressability. The system exhibits narrow ZPLs, spectral stability over more than an hour, spin-selective optical transitions and single-molecule optically detected magnetic resonance. Coherent control yields millisecond-scale dynamical-decoupling coherence and tens-of-milliseconds spin relaxation at a temperature of 4.5 K. These results establish molecular qubits as a viable platform for single-emitter quantum optics 5,6 while preserving the advantages of bottom-up chemical design 7 and processable materials 8–10 .