<p>The selective incorporation of single atoms into molecular frameworks remains a fundamental challenge in synthetic chemistry. While atom transfer reactions offer promising solutions, they require reagents with precisely tunable reactivity and excellent chemoselectivity. Herein, we report pyridinium ylides as photocatalytic oxygen atom and nitrogen group transfer reagents that combine a unique reaction mode with practical advantages including bench stability and operational simplicity. These ylides form reactive triplet diradicals via reversible photocatalytic activation, enabling selective formation of aziridines, epoxides, and oxo- and imino-transfer products at room temperature in open air. Their controlled reactivity allows selective transformations across diverse substrates, proving particularly valuable for late-stage functionalization. Mechanistic investigations combining experimental and computational methods reveal a triplet-triplet energy transfer pathway proceeding through a carbocation intermediate.</p>