Posterior tibial slope (PTS) has been known to contribute to anterior–posterior knee stability and play an essential biomechanical role in knee kinematics. This study aimed to investigate the effect of PTS on single-leg standing sagittal knee alignment of the intact knee. This study included 100 patients with unilateral ACL injury knee (ACL injury group, 53 patients) or with the normal knee (control group, 47 patients). The single-leg standing sagittal alignment of the unaffected knees of the ACL injury group and normal knees of the control group were assessed radiographically with the following parameters: knee extension angle (EXT), PTS, PTS to the horizontal line (PTS-H), femoral shaft anterior tilt to the vertical axis (FAT), and tibial shaft anterior tilt to the vertical axis (TAT). PTS was negatively correlated with EXT and positively correlated with TAT. EXT was significantly larger in the ACL injury group, whereas TAT was smaller in the ACL injury group. Patients with larger PTS tend to stand with a higher knee flexion angle by tilting the tibia anteriorly, possibly reducing tibial shear force. Patients with ACL injury tend to stand with larger EXT, i.e., there is less preventive alignment to minimize the tibial shear force.