BACKGROUND: The Hybrid Assistive Limb (HAL) is a robotic exoskeleton designed to support impaired limbs. OBJECTIVE: We aimed to evaluate whether active exercise using a single-joint HAL (HAL-SJ) following total knee arthroplasty can facilitate the recovery of knee flexion. METHODS: Twenty-two patients who underwent total knee arthroplasty were randomly allocated to the HAL-SJ group (n= 12) or conventional physical therapy (CPT) group (n= 10). On postoperative day 5, patients performed active knee flexion exercises either with or without HAL-SJ assistance every second day. Outcome measures included active and passive knee flexion range of motion (ROM), muscle strength, and pain intensity, as assessed by the visual analog scale, and were assessed on postoperative days 5 (pre-treatment) and 10 (post-treatment). Active ROM was measured at 6 months postoperatively; further long-term follow-up was performed by telephone interview. RESULTS: Both groups showed significant improvement between postoperative days 5 and 10 in all outcome measures. Improvements in active ROM (p< 0.01), passive ROM (p< 0.01), muscle strength (p< 0.01), and pain (p< 0.01) were significantly greater in the HAL-SJ group than in the CPT group. Long-term outcomes were also significantly better in the HAL-SJ group. CONCLUSIONS: HAL therapy enables patients to perform painless active movements and facilitates the recovery of knee function.