Cell encapsulation provides a chemical tool in the isolation and protection of living cells from harmful, and often lethal, external environment. Although several strategies are available to form nanometric shells on living cells, most methods heavily rely on the time-consuming multi-step processes. Here we report the interfacial supramolecular self-assembly of ferric ion (FeIII) and tannic acid (TA). The interfacial nanoshell formation is simple, fast, and, more importantly, cytocompatible. Its versatility is demonstrated with various interfaces: hollow microcapsules, encasing microbial or mammalian cells, are generated at the water-oil interface in the microfluidic device; a cytoprotective FeIII-TA shell forms rapidly on the surface of the alginate microbead that entraps probiotic Lactobacillus acidophilus; moreover, Saccharomyces cerevisiae, fed with FeIII, responds to TA in the outside medium and autonomously forms a pericellular FeIII-TA shell. This reaction system will advance chemical manipulability of living cells and also suggest a new structural motif in materials science.