Efficient emission of purely organic room‐temperature phosphorescence (RTP) is of great significant for potential application in optoelectronics and photobiology. Herein, we report an uncommon phosphorescent effect of organic single molecule enhanced by resulting supramolecular assembly of host–guest complexation. The chromophore bromophenyl‐methyl‐pyridinium (PY) with different counterions as guests display various phosphorescence quantum yields from 0.4 % to 24.1 %. Single crystal X‐ray diffraction results indicate that the chromophore with iodide counterion (PYI) exhibits the highest efficiency maybe due to the halogen‐bond interactions. Significantly, the nanosupramolecular assembly of PY chloride complexation with the cucurbit6uril gives a greatly enhanced phosphorescent quantum yield up to 81.2 % in ambient. Such great enhancement is because of the strict encapsulation of cucurbit6uril, which prevents the nonradiative relaxation and promotes intersystem crossing (ISC). This supramolecular assembly concept with counterions effect provides a novel approach for the improvement of RTP. An uncommon phosphorescent effect of an organic single molecule enhanced by supramolecular assembly of host–guest complexation is reported. The chromophore bromophenyl‐methyl‐pyridinium (PY) gives an enhanced phosphorescent efficiency of 81.2 % under ambient conditions after complexation with cucurbit6uril. The strict encapsulation of cucurbit6uril suppresses the nonradiative relaxation and promotes intersystem crossing.