<p>Red phosphorescent materials with long-lived triplet excitons are highly desirable for optoelectronic and bio-related applications but remain limited by short emission lifetime. Here we show ultralong red phosphorescent carbon nanodots, which exhibit minute-scale phosphorescence. Through combining excited-state engineering assisted by machine learning optimization, the obtained red carbon nanodots achieve the phosphorescence lifetime of 353 second, with emission persistence exceeding 4000 second. Spectroscopic analysis reveals the coexistence of a fast radiative transition channel from ³(<i>n</i>, <i>π</i>^*) to ¹<i>n</i>² and a slow transition channel from ³(<i>π</i>, <i>π</i>^*) to ¹(<i>π</i>, <i>π</i>^*), where the latter dominates the ultralong phosphorescence. As a proof of concept, a wearable passive light-emitting-diode indicator based on the carbon nanodots is demonstrated, enabling visual signaling in time-limited environments after power-off.</p>