The preparation of monodisperse polymer (polydopamine, PDA) capsules by a one-step interfacial polymerization of dopamine onto dimethyldiethoxysilane (DMDES) emulsion droplets and removal of the DMDES templates with ethanol is reported. The diameters of the PDA capsules can be tailored from 400 nm to 2.4 µm by varying either the DMDES emulsion condensation time or the emulsion concentration used for templating. Further, capsules with defined nanometer-scale shell thicknesses (ranging from ∼10 to 30 nm) can be prepared by adjusting the emulsion concentration. This shell thickness can be increased by repeated interfacial polymerization of dopamine, with three cycles yielding capsules with a shell thickness of up to 140 nm (for a 0.6% v/v suspension). Functional substances, such as organically stabilized magnetic (Fe3O4) nanoparticles, quantum dots (CdSe/CdS), and hydrophobic drugs (thiocoraline), can be preloaded in the emulsion droplets, and following PDA coating and DMDES removal, these materials remain encapsulated in the polymer capsules. All of the unloaded and loaded PDA capsules are monodisperse and do not aggregate. This work provides new avenues for the preparation of polymer capsules with defined size and shell thickness and for the encapsulation of a range of hydrophobic substances.