Background: Backward walking (BW) has been proven to reduce the external knee adduction moment (EKAM) and knee adduction angular impulse (KAAI) during gait in healthy subjects, but its effects in individuals with knee osteoarthritis (OA) remain unknown. This study aimed to investigate the effects of self-selected speed BW on the EKAM, KAAI, and external knee flexion moment (EKFM) in individuals with medial knee OA. Methods: Thirty-two participants with medial knee OA underwent a three-dimensional gait analysis across three randomized conditions: (1) self-selected speed forward walking (FW), (2) self-selected speed BW, and (3) speed-controlled forward walking (SCFW) (for each individual, the SCFW speed was controlled within a range of 95% to 105% of BW speed). For each condition, the first peak of EKAM, second peak of EKAM, first peak of EKFM, and the KAAI were determined. One-way repeated measures ANOVA and multiple pairwise comparisons were performed to compare peaks of EKAM, peak of EKFM, and the KAAI between conditions. Results: BW significantly reduced the first peak of EKAM and the KAAI in comparison with FW and SCFW (p < 0.001). Both BW and SCFW showed a significantly reduced first peak of EKFM in comparison with FW (p < 0.001). However, BW did not reduce the second peak of EKAM when compared with FW or SCFW (p > 0.05). Conclusions: BW can significantly reduce the first peak of EKAM and the KAAI in comparison with FW and SCFW in individuals with medial knee OA.