Organic molecules are attractive to both physicists and chemists because molecules could have high quantum efficiencies in light emission and be chemically synthesized to have transitions at desired wavelengths. In the past several decades, single molecules embedded in solids, as isolated individual quantum systems, have become an attractive class of sources of single photons since a single two-level system cannot emit two photons simultaneously, as each excitation and emission cycle requires a finite time^[1,2]. Single photons are one of the key building blocks for photonic quantum technologies, such as quantum computation, quantum key distribution, and metrology^[2–6]. Compared to various other solid-state single-photon emitters such as self-assembled quantum dots^[7–9], color centers in diamond^[10–12], and defects in two-dimensional materials^[13], single molecules possess several unique properties including small size of about one nanometer (suitable for high-density doping), flexibility in the synthesis, and strong and stable Fourier-transform-limited zero-phonon lines at low temperature. In particular, 7,8:15,16-dibenzoterrylene (DBT) molecules embedded in anthracene (AC) have been actively studied as definitely stable single-photon emitters with nonblinking emission^[13–16] and lifetime-limited linewidth^[17–19]. Recent reports have explored the integration of single DBT molecules with planar photonic circuits^[15,20–23]. However, despite enormous studies, the quantum efficiency of single DBT molecules in the AC matrix, as a critical piece of information, has not been experimentally measured.