Stimuli-sensitive polymersomes, composed of amphiphilic block copolymers, have emerged as a promising nanocarrier for triggered release of anticancer drugs. In this study, we synthesized a bioreducible, amphiphilic triblock copolymer based on poly(ethylene glycol)-b-poly(lysine)-b-poly(caprolactone) bearing a disulfide bond (PEG-b-PLys-SS-PCL). Owing to its unique amphiphilicity, the copolymer formed self-assembled polymersomes (256 nm diameter) under aqueous conditions. These polymersomes were stable in physiological solution (pH 7.4), whereas they readily disintegrated under a reductive environment similar to an intracellular condition. The polymersomes could simultaneously encapsulate the hydrophobic camptothecin (CPT) in their membrane and the hydrophilic doxorubicin·hydrochloride (DOX·HCl) in their aqueous cores. The polymersomes released the drugs in a sustained manner under physiological conditions (pH 7.4), whereas the drug release rates dramatically increased in a reductive environment at 10 mM glutathione. From in vitro cytotoxicity tests, it was found that dual drug-loaded polymersomes showed significantly higher cytotoxicity to SCC7 cancer cells than those with the single drug. These results suggest that the polymersomes bearing the bioreducible linker have high potential as carriers for intracellular dual-drug delivery.