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Photodynamic therapy (PDT) represents a promising strategy for cancer therapy. Nevertheless, the inherently hypoxic tumor microenvironment greatly impairs the therapeutic performance of conventional Type‐II PDT, which is strongly dependent on molecular oxygen. A biotin engineered phenothiazine photosensitizer is presented, which can generate type I reactive oxygen species through photoinduced electron transfer originating from its singlet excited state, thereby efficiently breaking the oxygen‐dependent bottleneck of conventional type II PDT. This photosensitizer inherits the excellent photophysical features of the phenothiazine scaffold and exhibits robust anticancer activity under both normoxic and hypoxic, validated by in vitro cell tests and in vivo assessments using subcutaneous tumor models in BALB/c mice. This study establishes a new strategy for constructing hypoxia‐resistant type I photosensitizers and supplies a promising candidate for PDT of hypoxic tumors. More details are discussed in the article by Yi et al . on pages 3581—3590. image