It is a significant challenge to achieve a high photoluminescence quantum yield of Cu(I) complexes, particularly for red Cu(I) emitters, because of their considerable reorganization energy and weak spin-orbit coupling. Herein, we report a red phosphorescent cyclic trinuclear Cu(I) complex (denoted as 1), which was formed by reacting alkyl-substituted pyrazole ligands (4-isobutyl-3,5-dimethyl-1H-pyrazole) with in situ generated Cu(I) ions. Complex 1 was nonemissive in a pure tetrahydrofuran (THF) solution, whereas it emitted intense red light when its molecules were aggregated by adding water to the THF solution. Near-unity photoluminescence quantum yields were achieved in a mixed solvent of THF/water (f= vol/vol, f= 90%), poly(methyl methacrylate) films, and in the solid state. Photophysical analysis and theoretical calculations revealed that the excellent photoluminescence performance was due to the aggregation-enhanced emission. In the aggregation state, cuprophilic interactions dominated the excited state, and the restricted movement of alkyl through CH···C/CH···πinteractions diminished nonradiative transition. A cyclic trinuclear Cu(I) complex was used to fabricate a nondoped organic light-emitting diode (OLED), which is the first instance of such Cu(I) materials.