Inspiration from the iron gall ink leads to the efficient formation of Fe(III)–tannic acid coacervate particles inside the phase-separated microdroplets that are derived from the aqueous PEG/dextran liquid–liquid phase separation system. This hierarchical self-assembly, in aid of the protein affinity of tannic acid, makes it possible to compartmentalize and condense proteins into a localized, compact space in the microdroplets.