Metal halide perovskites have attracted much attention in biomedicine because of their excellent fluorescence energy conversion properties; however, poor water-stability and cytotoxicity limit its applications as a biomedical tracer, especially in cellular imaging. Herein, water-ultrastable perovskites CsPbBr 3 :Cs 4 PbBr 6 nanocrystals (NCs) encapsulated in chitosan are fabricated successfully using a water-triggered method. The as-synthesized CsPbBr 3 :Cs 4 PbBr 6 @CS (chitosan, CS) nanoparticles in water display enhanced fluorescence emission for 35 days. Further, the viability of glioma cells (U87 cells) incubated with different concentrations of CsPbBr 3 :Cs 4 PbBr 6 @CS nanoparticles (0–20 μg·ml −1 ) for 24 h is found to be higher than 90%. In artificial body fluid, analyses using laser confocal microscopy, the standard Cell Counting Kit-8 (CCK-8) method, and flow cytometry demonstrated the good water ultrastability and high biocompatibility performance of CsPbBr 3 :Cs 4 PbBr 6 @CS nanoparticles in cellular imaging. Overall, the water-ultrastable halide perovskites support promising perspectives in biological cell tracing and intelligent medical technology. Graphical Abstract