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Hypoxia is a critical factor for restricting photodynamic therapy (PDT) of tumor, and it becomes increasingly severe with increasing tissue depth. Thus, the relief of deep tumor hypoxia is extremely important to improve the PDT efficacy. Herein, tumor microenvironment (TME)‐responsive size‐switchable hyaluronic acid‐hybridized Ru nanoaggregates (HA@Ru NAs) were developed via screening reaction temperature to alleviate deep tumor hypoxia for improving the tumor‐specific PDT by the artful integration multiple bioactivated chemical reactions in situ and receptor‐mediated targeting (RMT). In this nanosystem, Ru NPs not only enabled HA@Ru NAs to have near infrared (NIR)‐mediated photothermal/photodynamic functions, but also could catalyze endogenous H2O2 to produce O2in situ. More importantly, hyaluronidase (HAase) overexpressed in the TME could trigger disassembly of HA@Ru NAs via the hydrolysis of HA, offering the smart size switch capability from 60 to 15 nm for enhancing tumor penetration. Moreover, the RMT characteristics of HA ensured that HA@Ru NAs could specially enter CD44‐overexpressed tumor cells, enhancing tumor‐specific precision of phototherapy. Taken together these distinguishing characteristics, smart HA@Ru NAs successfully realized the relief of deep tumor hypoxia to improve the tumor‐specific PDT. [ABSTRACT FROM AUTHOR]