A recyclable peroxidase mimic Fe₃O₄@polydopamine/Prussian blue (Fe₃O₄@PDA/PB) composite was facilely prepared by coating PDA on an Fe₃O₄ nanoparticle core and in situ growth of PB nanoparticles on a PDA shell. The prepared Fe₃O₄@PDA/PB composite exhibited excellent peroxidase-like activity and can catalytically oxidize the colorless colorimetric substrate 3,3′,5,5′-tetramethylbenzidine (TMB) into a blue colored product in the presence of H₂O₂ at 30 °C in 1 min. The catalytic mechanism was deduced to be the nanozyme-promoted generation of a hydroxyl radical (·OH), and the catalytic behavior followed the typical Michaelis–Menten kinetics. Based on Cr(VI)-boosted peroxidase-like activity of Fe₃O₄@PDA/PB, a simple and fast colorimetric method for detection of Cr(VI) was developed. Under the optimum conditions, the colorimetric method exhibited wider linear range (100 nM to 140 μM), low LOD (51.1 nM), good selectivity and short detection time (1 min). Moreover, the feasibility of the proposed colorimetric method was evaluated by determination of Cr(VI) in spiked tap water and lake water samples. Good recoveries (95.2–102.9%) and low relative standard deviations (RSDs) (1.6–4.4%) were obtained, showing great promise for practical use.