Although plastics have benefited our lives in terms of cost and convenience, the disposal of end-of-life plastics poses environmental problems, such as microplastics (MPs). Although the separation (e.g., filtration) and staining of MPs with fluorescent dye/solvent are generally accepted steps to observe MPs in an environmental matrix, in this study, an in situ selective fluorescent illumination of the MPs in water was attempted with the aid of surfactant. Nonpolar fluorescent dye in combination with surfactant affords nanometer-sized dye particles in water, which adsorb on MPs and penetrate the polymer matrix for effective staining and stable fluorescent behaviors. The effects of different staining parameters, including different dyes, surfactants, staining temperatures, staining times, dye/surfactant ratios, dye/MP ratios, and MP concentrations in aqueous solutions were investigated to better understand staining conditions. More interestingly, non-adsorbed free dye molecules in the staining solution were almost completely fluorescence-quenched by introducing the quenching agent, aniline, while the fluorescence intensity of the stained MP was maintained. By staining MPs with a dye/surfactant combination and subsequently quenching with aniline, in situ selective fluorescent illumination of the MPs in water was successfully achieved, which may eliminate the tedious separation/filtration procedure of MPs to accomplish the quick detection or monitoring of MPs.