Cavity ring-down spectroscopy (CRDS) offers exceptional sensitivity for trace-gas detection, yet its application to quantitative tritium analysis has remained largely unexplored. In this study, we demonstrate mid-infrared CRDS targeting the fundamental \(\nu _{1}\) vibrational band of tritiated water (HTO) near \({4.3}\,{\upmu }\) m. By exploiting this strong fundamental absorption, the system achieves substantially improved detection sensitivity of HTO compared with previous optical approaches. For the first time, a proportional response to tritium activity was experimentally verified with high linearity ( \({R}^{2}\) > 0.99) and excellent repeatability was demonstrated. Practical detection limits were rigorously assessed by accounting for sample introduction and utilization efficiency, enabling a realistic performance evaluation under operational conditions. The system enables quantitative measurements of microliter-scale samples and offers potential for rapid tritium assessment in fusion and accelerator facilities. These results establish a scalable spectroscopic framework for tritium analysis and provide a pathway toward future ultra-high-sensitivity implementations based on advanced CRDS architectures.
