Abstract Optical bioimaging is an ever‐growing field that benefits both from the fast progress of optical instrumentation and modalities, and from the development of light‐emitting probes. The efficacy of molecular fluorescent dyes is crucial, yet hindered by limited brightness and hydrophilicity. Addressing these challenges, self‐stabilized fluorogenic organic nanoparticles only made of pure dyes ( d FONs) are introduced in this work. Comprising thiol‐sensitive fluorogenic chromophores, these d FONs exhibit enhanced brightness exclusively in the presence of biological thiols, notably glutathione, overcoming the need for water‐solubilizing moieties. Importantly, these nanoparticles demonstrate large fluorescence and one‐ and two‐photon brightness, enabling sensitive bioimaging of intracellular thiols at micromolar concentrations. Notably, only the pristine fluorogenic nanoparticles can penetrate the cells and does not require to wash the cells before imaging, emphasizing their unique role as dye carriers, fluorogenic probes and ease of use. This work highlights the transformative potential of d FONs in advancing optical bioimaging, paving the way for the use of d FONs not just as tracers, but also now as biosensors and ultimately in the future as biomarkers.