We present the preparation of metastable single-chain polymeric nanoparticles through stimuli-responsive and reversible intramolecular cross-linking of a single polymer chain in water. Functionalisation of poly(N-hydroxyethylacrylamide) polymers with viologen, followed by addition of nor-seco-cucurbit[10]uril allows for facile preparation of nanoparticles through specific and tightly binding 2 : 1 homo-ternary complexes. Particle sizes can be readily tuned through either polymer molecular weight or degree of cross-linking. By virtue of the supramolecular nature of the intramolecular cross-linking in water, the formation of these metastable nanoparticles represents a step towards the development of synthetic polymers with aqueous phase behaviour analogous to the self folding of biomacromolecules.