Pure organic room temperature phosphorescence (RTP) is highly preferable because of its long lifetime and potential applications. However, these kinds of materials are still very scarce due to the weak spin–orbit coupling between singlet and triplet states and easily nonradiative decay of the excited states. Achieving room temperature phosphorescence under visible light excitation is particularly challenging in aqueous solution. Herein, a micelle‐assisted assembling strategy has been developed to realize pure organic RTP in water by using donor–acceptor molecules. A visible‐light responsive long‐lived RTP in water with a lifetime more than 3 ms is obtained by the prepared nanocrystals. However, the same molecules show no RTP as rigid bulk crystals. Spectroscopic studies, single‐crystal structure analysis, X‐ray diffraction patterns, and density functional theory calculations reveal that the intermolecular interactions, heavy atom effect, and the molecular packing way play critical role to the long‐lived RTP character for the assembled nanocrystals in water and thermally activated delayed fluorescence for crystals in solid. A micelle‐assisted assembly strategy to realize long‐lived pure organic room temperature phosphorescence under visible light excitation in water is reported. The molecular packing plays a critical role to the room temperature phosphorescence character in the assembled nanocrystals and delayed fluorescence in the crystals.