Context: Previous studies have reported that the incidence of patellofemoral pain in women is 2.2 times higher than that in men. Lower hip frontal dynamic joint stiffness in women may be related to the magnitude of hip adduction and internal rotation associated with patellofemoral pain. Objective: To identify sex differences in hip frontal dynamic joint stiffness and examine the relationship between hip frontal dynamic joint stiffness and hip adduction and internal rotation during gait. Design: Cross-sectional study. Setting: University campus. Participants: A total of 80 healthy volunteers (40 women and 40 men) participated in this study. Intervention(s): Kinematic and kinetic data during gait were collected using a motion capture system and force plates. Main Outcome Measures: Hip frontal dynamic joint stiffness, hip adduction, and hip internal rotation were calculated during gait. Results: Women demonstrated lower hip frontal dynamic joint stiffness than men during gait (P < .01). They also displayed decreased hip frontal dynamic joint stiffness associated with increased hip adduction (r = −.85, P < .001) and internal rotation (r = −.48, P < .001). Conversely, in men, decreased hip frontal dynamic joint stiffness was associated with increased hip adduction (r = −.74, P < .001) but not internal rotation (r = .17, P = .28). Conclusions: Sex differences between hip frontal dynamic joint stiffness and hip internal rotation during gait may contribute to the increased incidence of patellofemoral pain in women.