Airborne singlet oxygen (1O2) molecules, which are generated during the TiO2 photocatalytic reactions and diffused from the surface into air, were detected at the opposite surface using terrylenediimide (TDI) molecules at the single-molecule level. The novel 1O2 nanosensor, which has a detectable number of about 1000 1O2 molecules in 70 x 70 square micrometers, can easily detect the single 1O2 molecule at a distance of over 1000 micrometers from the place of its creation in ambient air.