Photoswitchable polymer nanoparticles (ps-PNPs) have been constructed by emulsion polymerization through embedding an iridium(Ⅲ) complex, [Ir(bt)₂(pic)] (bt = phenylbenzothiazole, pic = picolinate), as a fluorophore and a diarylethene derivative (DTE) as a photochrome. The ps-PNPs show reversible luminescence modulation with DTE switching using UV light in aqueous solution and even in living cells. Such luminescent modulation only occurred in polymer nanoparticles and not in a simple mixed solution with low concentrations of [Ir(bt)₂(pic)] and DTE. The ps-PNPs displayed good cellular uptake and fast intracellular luminescence modulation by photoswitching, indicating the nanoparticles could be developed as photoswitchable luminescent probes for a diversity of bioimaging applications.