A facile synthesis of highly stable, water-dispersible metal-nanoparticle-decorated polymer nanocapsules (M@CB-PNs: M = Pd, Au, and Pt) was achieved by a simple two-step process employing a polymer nanocapsule (CB-PN) made of cucurbit [6]uril (CB[6]) and metal salts. The CB-PN serves as a versatile platform where various metal nanoparticles with a controlled size can be introduced on the surface and stabilized to prepare new water-dispersible nanostructures useful for many applications. The Pd nanoparticles on CB-PN exhibit high stability and dispersibility in water as well as excellent catalytic activity and recyclability in carbon-carbon and carbon-nitrogen bond-forming reactions in aqueous medium suggesting potential applications as a green catalyst.Metal nanoparticles (NPs) have attracted great attention because their unique properties such as high surface to volume ratio, quantum confinement, and surface plasmon effect can contribute to diverse applications in catalysis, nanoelectronics, molecular imaging, biosensors, and nanomedicine. [1] Many of these intriguing properties strongly depend on the size [2] and surface area [3] of the NPs. In addition, support materials (or stabilizers) [4] such as poly-mers, [5] dendrimers, [6] silica, [7] and metal oxides, [8] which are required for some applications (e.g. heterogeneous catalysis), also affect the properties of NPs. Although NPs on solid supports have been successfully employed in catalysis, most systems suffer from shortcomings such as passivation of the NP surface, [9] lack of long-term stability, [10] low dispersibility, [11] deactivation or constant leaching, [12] and low recyclability which limit their capacity and applications. [13] For example, NPs supported on mesoporous/modified silica are unstable, which leads to a rapid decay of the catalytic activity under the reaction conditions. [12] Furthermore, the catalytic activity and stability of NPs in environmentally benign media such as water has important environmental, economical, and safety implications, which are crucial in green chemistry. [14] However, to date, catalysis using NPs on solid supports has seldom been explored in water. [15] Cucurbit[n]uril (CB[n]: n = 5-8, 10) exhibits remarkable recognition abilities with high affinity and selectivity towards organic and inorganic species [16] and stabilizes NPs by forming a passive layer or acting as a protecting agent. [17] We recently reported hollow polymer nanocapsules (CB-PNs) with a thin shell composed of covalently linked cucurbit[6]uril (CB[6]) units, [18] which can be readily synthesized from commercially available allyloxyCB[6] and dithiol (3,6-dioxa-1,8-octanedithiol) in a one-pot reaction. Unique features of CB-PNs include facile tailoring of their surface to prepare new functional materials for various applications such as targeted drug delivery, diagnosis, and imaging. [18c,d] Moreover, CB-PNs synthesized in the presence of excess dithiol have "disulfide loops" protruding from the surface, [18a,b, 19] which can be used as an ...