BACKGROUND: Individuals with chronic ankle instability (CAI) may experience recurrent ankle sprains and symptoms during daily activities such as stair descent, where the associated proprioceptive deficit is largely unevaluated. OBJECTIVES: To evaluate the reliability and validity of an ankle inversion discrimination apparatus for stair descent, and examine whether proprioceptive scores from this apparatus are associated with patient-reported symptoms. DESIGN: Cross-sectional study. METHOD: Sixty-six participants volunteered in this study. The ankle inversion discrimination apparatus was purpose-built to assess ankle proprioception across four positions of ankle inversion (10°, 12°, 14°, and 16°) during stair descent. The Area Under the Receiver Operating Curve (AUC) was employed as the ankle proprioceptive discrimination score. RESULTS: Test-retest reliability ICC (3,1) for the whole group was 0.825, with 0.747 for the non-CAI group (95%CI = 0.331-0.920) and 0.701 for CAI (95%CI = 0.242-0.904). The CAI group performed at a significantly lower level than non-CAI on the ankle inversion discrimination apparatus for stair descent assessment (0.769 ± 0.034 vs. 0.830 ± 0.035, F = 33.786, p < 0.001). CAIT scores were strongly and significantly correlated with scores from this apparatus (Spearman's rho = 0.730, p < 0.001). CONCLUSIONS: The ankle inversion discrimination apparatus for stair descent is reliable and valid for assessing task-specific ankle proprioceptive impairments in CAI. The strong and significant relationship found between ankle proprioception during stair descent and the severity of CAI suggests that rehabilitation programs focusing on deficits in ankle inversion proprioception during stair descent may improve self-reported instability in CAI.