A new approach to prepare functional organic nanoparticles (ONPs) from linear polymers is described. The nanoparticles are obtained by ring-opening metathesis polymerisation (ROMP) of functionalised norbornene dicarboximides, side-chain amidation with tri-O-allyl-TRIS, and ring-closing metathesis (RCM). The synthesis is quite flexible and mild, allowing preparation of organic- and aqueous-soluble particles with narrow molecular weight (MW) distributions that are tunable, ranging from MW ≈ 10 to >100 kD with diameters between ca. 5 and 50 nm. The use of functional monomer(s) and/or chain-transfer agents (CTAs) leads to the fully controlled synthesis of nanoparticles containing single or multiple reactive functional groups. This non-toxic ONP scaffold can be readily used as a protective carrier for a broad range of groups using the appropriate monomers, such as a fluorescein-functionalised monomer that affords fluorescent ONPs with significantly enhanced photostability.