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Protein kinases constitute a rich pool of biomarkers and therapeutic targets of tremendous diseases including cancer. However, sensing kinase activity in vivo while implementing treatments according to kinase hyperactivation remains challenging. Herein, we present a nanomediator–effector cascade system that can in situ magnify the subtle events of kinase‐catalyzed phosphorylation via DNA amplification machinery to achieve kinase activity imaging and kinase‐responsive drug release in vivo. In this cascade, the phosphorylation‐mediated disassembly of DNA/peptide complex on the nanomediators initiated the detachment of fluorescent hairpin DNAs from the nanoeffectors via hybridization chain reaction (HCR), leading to fluorescence recovery and therapeutic cargo release. We demonstrated that this nanosystem simultaneously enabled trace protein kinase A (PKA) activity imaging and on‐demand drug delivery for inhibition of tumor cell growth both in vitro and in vivo, affording a kinase‐specific sense‐and‐treat paradigm for cancer theranostics. [ABSTRACT FROM AUTHOR]