Over the past few decades, novel fluorescent nanoprobes have consistently attracted a great research interest due to their superior optical characteristic. Based on their compositions, these fluorescent nanoparticles can be mainly divided into fluorescent inorganic nanoparticles (FINs) and fluorescent organic nanoparticles (FONs). A prospective nanoprobe should have a suitable size and size distribution, high water dispersibility, good biocompatibility and biodegradability. Although FINs possess remarkable fluorescence properties and have been widely explored for bioimaging applications, most of the FINs are non-biodegradable, in addition, some of them have shown a fetal toxicity to living organisms. In this work, we reported that novel fluorescent polydopamine nanoparticles (FPNPs) can be conveniently fabricated via the diamine molecules-induced degradation of as-prepared non-fluorescent polydopamine (PDA) microparticles at room temperature. The FPNPs exhibited strong green fluorescence, high water solubility and good biocompatibility, fabricating them highly potential for biological imaging application. More importantly, we firstly found that FPNPs selectively label neuromast hair cells, their applications as a reliable fluorescent indicator to investigate the neuromast hair cells in the lateral line of zebrafish, to in turn determine the viability of hair cells, was demonstrated.