Purpose <p>To evaluate the dynamics of the talus–lateral malleolus distance during landing and compare the results between individuals with chronic ankle instability (CAI) and healthy adults.</p> Methods <p>Participants were divided into the healthy (12 adults, 12 feet) and CAI (12 adults, 12 feet) groups. Participants performed forward single-leg drop landing from a 30-cm-high box. Through the synchronization of ultrasound with a motion capture system, measurements of the talus–lateral malleolus distance and ankle joint angles were obtained during forward single-leg drop landing. The analysis interval was from 200&#xa0;ms before to 200&#xa0;ms after initial contact; the time was normalized to 100%. Statistical parametric mapping was employed to investigate differences in the temporal changes of the talus–lateral malleolus distances and ankle joint angles during landing between groups.</p> Results <p>A significantly longer talus–lateral malleolus distance was observed before initial contact in the CAI group (1–49%) as compared with the healthy group (<i>p</i> = 0.000). In addition, the ankle adduction angle before initial contact (12–61%) was larger in the CAI group than in the healthy group (<i>p</i> = 0.005).</p> Conclusion <p>Our findings suggest that the talus–lateral malleolus distance increases along with the ankle adduction movement in individuals with CAI, especially before initial contact, potentially indicating high mechanical ankle instability. This study established an innovative measurement system for quantitative evaluation of the dynamics of the talus and lateral malleolus during landing. The findings of this study may contribute to a more detailed understanding of pathology and features of movement in patients with CAI.</p>