Marine micro/nanoplastic (MNP) contamination has evolved into a serious threat to the ecosystem. Unfortunately, MNPs can act as vectors for heavy metals (e.g., Pb(II)), which can exacerbate their ecotoxicological effects. The development of MNPs removal methods, as well as, MNP surface clean-up strategies is, therefore, crucial to remediating marine MNP pollution. Herein, we report a magnetic “nano-fishnet” (i.e., alkylated nanoscale zerovalent iron grown on naturally derived cellulose nanofibers (ac-nZVI)) to quickly enrich and trap MNPs from seawater. Driven by the attractive electrostatic and hydrophobic interactions of ac-ZVI, the removal efficiency of 100 nm, 2 μm and 10 μm MNPs (PS, PVC, PMMA) from artificial seawater (ASW) has been shown to reach 98.28%, 98.58% and 97.64% within 1 min, with the calculated collection capability of 40.96, 167.62 and 324.88 mg_MNPs g_Fe⁻¹, respectively. Similarly, its rapid and complete removal performance has also been demonstrated in MNP-spiked natural seawater. Furthermore, the microscopic reaction behaviors between ac-nZVI and MNPs with different sizes are unraveled; the mechanism involves the encapsulation of single plastic spheres, adhesion of multiple particles and trapping of larger aggregates. After the reaction, the MNPs are packaged and removed using a magnet. Moreover, the unique core–shell structure endows the iron nanoparticles with rich aquatic chemistry properties for Pb(II) transformation and detoxification. The full contact between ac-nZVI and MNPs facilitates the reduction reactions between Fe⁰ and Pb(II); thus, Pb(II) on the MNP surface is reduced to Pb⁰. Our work suggests an appealing strategy for the remediation of MNP-polluted seawater and proposes a new concept for the toxin decontamination of MNPs.