beta-Sheet crystals in natural silks are particulate and less than 10 nm in size in all three dimensions. In synthetic supramolecular analogues of natural silks, beta-sheet crystals have been found to be fibrous with the longest dimension exceeding 100 nm in the hydrogen-bonding direction. This work demonstrates that particulate beta-sheet crystals can be achieved without the use of an elaborate amino acid sequence by simply grafting oligo(beta-alanine) segments as pendent side groups to a butyl rubber main chain. The size control in the hydrogen-bonding direction is attributable to an entropic force that opposes the driving force for the self-assembly. The nanocrystals, especially those of trimeric beta-alanine segments, display a remarkable ability to simultaneously provide stiffness, extensibility, and strength to the synthetic elastic network and do so highly efficiently at a low volume fraction of the material. The herein studied butyl rubber-based thermoplastic elastomers containing no more than 3.6 vol % of beta-sheet nanocrystals are stiffer, stronger, and more extensible than vulcanized butyl rubber reinforced by 20 vol % of carbon black and poly(styrene-b-isobutylene-b-styrene) reinforced by >33 vol % of polystyrene domains. The high reinforcing efficacy of the beta-sheet crystals is attributable to two phenomena associated with their small sizes: a stick slip mechanism for energy dissipation and an auxiliary layer of polymer brush that contributes to increasing the modulus.