"Harnessing the catalytic power of heme catalysts for advanced synthesis, particularly in environmentally benign aqueous media, remains a significant challenge due to hemin's poor solubility and the need for precise microenvironment control. Here, the study reports a tailored amphiphilic random-block copolymer, poly(histidine-random-styrene)-block-poly(N,N-dimethylacrylamide) (P(His-r-Sty)-b-PDMAA), designed as an ""all-in-one"" nanocarrier for efficient aqueous hemin catalysis. Synthesized via controlled RAFT polymerization, this copolymer self-assembles into stable nanomicelles featuring a hydrophobic core functionalized with histidine residues for specific axial coordination of hemin, and a hydrophilic shell ensuring aqueous compatibility. This precisely engineered architecture enables the highly efficient cyclopropanation of styrene with ethyl diazoacetate in water using only 0.3 mol% hemin loading, achieving yields up to 94.4% under ambient air. The catalytic system demonstrates robustness and sustainability, retaining over 90% activity after four cycles via a simple extraction-based recovery. Thus, the work showcases a powerful strategy integrating controlled polymer synthesis and bio-inspired ligand design to create highly active and recyclable catalysts for challenging transformations in water."