Background: A safe and simple procedure to evaluate functional instability due to anterior cruciate ligament (ACL) deficiency (ACLD) has not been established. The angle of trunk backward tilting, which is known as a posture at risk for ACL injuries, could be used as a parameter to evaluate functional instability due to ACLD. Purpose: To measure the backward tilt angle of the trunk with participants standing upright on 1 leg and to investigate its usefulness to quantitatively evaluate functional instability due to ACLD. Study Design: Cohort study (diagnosis); Level of evidence, 3. Methods: Our cohort included 50 participants with unilateral ACLD and 40 participants with bilateral healthy knees. The trunk backward tilt (TBT) test was conducted as follows: the participant was asked to maximally tilt the trunk backward in a single-leg standing position, while forward tilt of the index leg was blocked with a custom-made device. The TBT angle was measured using a side-view photograph. Subjective knee instability during the test was recorded using a visual analog scale (VAS). The relative and absolute reliability of the TBT test were verified in a sample of healthy participants and those with ACLD, and comparisons between indicators were performed. Multiple regression analysis was performed with the injured/uninjured side ratio (I/U ratio) of the TBT angle as the dependent variable and the following independent variables: (1) I/U ratio of knee extension muscle strength, (2) I/U ratio of knee flexion muscle strength, (3) side-to-side difference (SSD) of the KT-1000 arthrometer measurement, (4) sex, and (5) SSD of the VAS score. Results: The TBT test had high reliability among healthy participants and those with ACLD. The TBT angle was significantly smaller and the VAS score was significantly higher on the injured side compared with the uninjured side and with healthy knees (P < .001 for all). Among the independent variables, the SSD of the VAS score had a negative influence on the I/U ratio of the TBT angle (R2 = 0.59; P < .001). Conclusion: The TBT test is a simple, safe, and reliable method for quantitatively evaluating functional instability due to ACLD under weightbearing conditions that reflect subjective knee instability. The test will provide an index of treatment outcomes and return to sports through additional objective measurements before and after ACL reconstruction.