Stimulated emission depletion (STED) nanoscopy is a promising super-resolution imaging technique for microstructure imaging; however, the performance of super-resolution techniques critically depends on the properties of the fluorophores (photostable fluorophores) used. In this study, a suitable probe for improving the resolution of STED nanoscopy was investigated. Quantum dots (QDs) typically exhibit good photobleaching resistance characteristics. In comparison with CdSe@ZnS QDs and CsPbBr3 QDs, Cd-free InP/ZnSeS QDs have a smaller size and exhibit an improved photobleaching resistance. Through imaging using InP/ZnSeS QDs, we achieved an ultrahigh resolution of 26.1 nm. Furthermore, we achieved a 31 nm resolution in cell experiments involving InP/ZnSeS QDs. These results indicate that Cd-free InP/ZnSeS QDs have significant potential for application in fluorescent probes for STED nanoscopy.