The current challenge in the field of fluorescent nanoparticles (NPs) for bioimaging is to achieve extreme brightness and external control of their emission using biodegradable materials. Here we propose a new concept of fluorescent polymer NPs, doped with ionic liquid-like salts of a cationic dye (octadecyl rhodamine B) with a bulky hydrophobic counterion (fluorinated tetraphenylborate) that serves as spacer minimizing dye aggregation and self-quenching. The obtained 40-nm poly(D,L-lactide-co-glycolide) NPs containing up to 500 dyes are brighter than quantum dots and exhibit photo-induced reversible on/off fluorescence switching, never reported for dye-doped NPs. We show that this collective switching of hundreds of dyes is due to ultrafast excitation energy transfer and can be used for super-resolution imaging. These NPs, being spontaneously endocytosed by living cells, feature high signal-to-noise ratio and absence of toxicity. The counterion-based concept opens the way to a new class of nanomaterials for sensing, imaging and light harvesting.