Electron transfer (ET) is crucial in many chemical reactions, but its mechanism and role are hardly understood in nanobiotechnology due to the complexity of reaction species and pathways involved. By modulating and monitoring electron behavior at the single-molecule level, we can better understand the fundamental mechanisms and ways to control them for technological use. Here, we unravel a mechanism of single-electron catalysis under positively charged nanoconfinement. We demonstrate that both (2 + 2) and (4 + 4) cycloadditions can be catalyzed reversibly by a single electron. Key reaction pathways are discovered by monitoring sequential electrical signals in the cycloadditions through advanced single-molecule detection platforms. Experimental and theoretical results consistently demonstrate that combining single ET processes with nanoconfinement involving cucurbit[8]uril can lower the reaction energy barrier and promote reversible cycloaddition. Moreover, we show that the bias voltage can fine-tune ET processes and chemical equilibria in bond formation and cleavage. Our results provide a novel approach to elucidate, modulate, and design electron-involved reactions and functionalized devices.