A facile approach for the preparation of supramolecular polymer-based fluorescent nanoparticles (FNPs) is reported. FNPs with homogeneous shape and size distribution are fabricated from low-molecular-weight molecules, and thus, different compositional constituents can be efficiently incorporated via copolymerization. The emission color of the FNPs covers a wide region from blue to near infrared and can be easily tuned using efficient excitation energy transfer. The photoswitchable fluorescent nanoparticles with high on-off fluorescence contrast are also simply prepared by copolymerization of monomers containing a fluorophore and a photochromic unit. Our FNPs are successfully applied in living cell imaging and as fluorescent inks.