Objective To investigate the effect of tritiated water on the immune system of zebrafish and its potential molecular mechanism. Methods Zebrafish embryos (2.5 to 3 hours post-fertilization [hpf]) were exposed to 3.7 × 10⁴ Bq/mL tritiated water (tritiated water group), and those exposed to E3 culture medium were used as the control group. The mortality rate, hatching rate, deformity rate, heart rate, body length, yolk sac area, neutrophil count in the tail, immune-related gene expression, and immune-related protein expression of zebrafish in the two groups were determined. Then transcriptome technology was used to further analyze the possible mechanism of tritiated water affecting the immune system of zebrafish. Results Compared with the control group, zebrafish at 72 hpf in the tritiated water group had no significant changes in the mortality rate, hatching rate, deformity rate, body length, and yolk sac area((t = 0.9045, 0.5000, 1.0000, 0.7238, 0.0337, P = 0.4169, 0.6433, 0.3739, 0.4785, 0.9735), but had significantly increased heart rate(t = 4.575，P = 0.002). At 4 days post-fertilization (dpf), the neutrophil count in the tail of zebrafish in the tritiated water group was significantly increased(t = 2.563，P = 0.0196), the mRNA expression of TNF-α was significantly decreased(t = 2.891, P = 0.045), the protein expression of nuclear factor-kappa B (NF-κB) was significantly increased(t = 3.848, P = 0.018), and the protein expression of NLRP3 was significantly decreased(t = 14.98, P = 0.001). At 7 dpf, the neutrophil count in the tail and the protein expression levels of NF-κB, NLRP3, and interleukin-1β were significantly decreased(t = 3.772, 7.048, 15.620, 4.423, P = 0.014, 0.002, 0.0001, 0.012). Transcriptome sequencing revealed that differentially expressed genes were mainly enriched in the “neutrophil activation” and “platelet activation pathways” at 4 dpf and in the “neutrophil apoptosis”, “ferroptosis”, and “necroptosis” pathways at 7 dpf. Conclusion Tritiated water exposure induces a temporally dynamic immune response in zebrafish, potentially affecting immune homeostasis by regulating neutrophil activation and apoptosis, as well as the expression of NF-κB and NLRP3.