Polymer-shelled vesicles are prepared by using cyclodextrin vesicles as supramolecular templates and an adamantane-functionalized poly(acrylic acid) additive anchored via host-guest recognition, followed by cross-linking of carboxylic acid groups on the polymer. The polymer-shelled nanocontainers are highly stable and effective for encapsulating small hydrophilic molecules. We also show that a hollow cross-linked polymer cage can be obtained after dissolution of the template vesicles. The size and shell thickness of the polymer cage can be tuned by variation of template size and polymer length.