Afterglow or persistent luminescence eliminates the need for light excitation and thus circumvents the issue of autofluorescence, holding promise for molecular imaging. However, current persistent luminescence agents are rare and limited to inorganic nanoparticles. This study reports the design principle, synthesis, and proof-of-concept application of organic semiconducting nanoparticles (OSNs) with ultralong phosphorescence for in vivo afterglow imaging.