[Purpose] The purpose of this study was to clarify the influence of trunk posture and gender on the knee joint angle and muscle activity during single-leg landing. [Subjects and Methods] Forty healthy college students were enrolled in the present study. The participants performed single-leg landing from a 40 cm-high platform using one of the three trunk postures: preferred, flexion, and right lateral flexion. The knee flexion and valgus angle was measured, and the hamstrings/quadriceps ratio (HQR) was calculated for each trunk posture. [Results] The knee flexion angle was significantly greater in the flexed posture, whereas the knee valgus angle was significantly greater in the right lateral flexed posture. In addition, the knee valgus angle was significantly higher in females, and the HQR was significantly lower in females than in males. [Conclusion] Both the trunk posture and gender affected the knee angle and muscle activity during single-leg landing. These findings are likely related to the differences in the incidence of anterior cruciate ligament injury between males and females.