The preparation of anisotropic naoprticles as drawn much attention in the literature, with most,:of the efforts being dedicated to convex particles. In this work, instead we present a reliable method to synthesis silica nanobowls with one well-defined opening, coveting a broad range of sizes: The nandbowls have been obtained from asymmetrically funetionalized silica polymer Janus nanodumbells, used as templates, by removing of the polymer. Polystyrene seeds having different sizes. as-Well as surface chemistry have been used as starting material in a two-step seeded emulsion polymerization, which leads to polymer nanodumbbells. These dumbbells are also asymmetrically functionalized due to the presence of silane groups on only one of their two hemispheres. This allows us to selectively coat the silane-beating hemisphere of the dumbbells with a silica layer by means of a Stoeber process. The silica nanobowls are eventually obtained after either calcination or dissolution of the polymeric template. Depending on the route followed to remove the polymer, nanobowls made of pure silica (from calcination) or hybrid Janus nanobowls with a silica outer shell and a covalently bound hydrophobic polymer layer inside the cavity (from dissolution) could be prepared. The difference :between the two types of nanobowls has been proved by electrostatically binding oppositely charged silica nanoparticles, which adhere selectively only on the otter silica part Of the nanobowls prepared by polymer dissolution, while they attach both inside and outside of nanobowls prepared by,calcination. We also show that selective functionalization of the outer surface of the Janus nanobowls from dissolution is possible. This work is one of the first examples of concave:, objects bearing different functionalities in the inner and outer parts of their surface.