Knee osteoarthritis (OA) often occurs in older women. Walking assistance such as knee brace is used to reduce mechanical stress on the knee, preventing OA onset. Dynamic joint stiffness (DJS) quantifies the resistance of an assistive device, providing a foundation for an objective bending stiffness prescription model. DJS may show sex differences among older adults. This study aimed to investigate sex differences in lower limb DJS in the sagittal plane during walking in older adults. A total of 132 healthy older adults, aged 65 years or older (71 men and 61 women), were extracted from a public dataset. DJS of the hip, knee, and ankle joints in the sagittal plane was determined during the power absorption phase of the stance. DJS, joint angular excursion, and Δ joint moment were compared between older men and women using the Mann-Whitney U test. In addition, the r-value was calculated to represent the effect size of the differences in amplitude. Ankle DJS in older women was significantly lower with a reduced Δ ankle plantar flexion moment compared with that into men (p < 0.001 and p = 0.001; r = 0.35 and 0.42, respectively). Additionally, knee DJS was lower in older women (p = 0.007). However, since the joint angular excursion and ΔMoment showed no differences (p = 0.624 and 0.222, respectively), the effect size was small (r = 0.24). Hip DJS showed no significant sex differences (p = 0.703). These results suggest that the decrease in ankle DJS in older women was caused by the reduced ankle plantarflexion moment. Thus, support for ankle DJS is necessary for healthy older women. Nonetheless, knee DJS does not elucidate the cause of knee OA in the older women.