Introduction The first weeks after total knee arthroplasty (TKA) are crucial for the functional outcome. To improve knee mobility, a continuous passive motion (CPM) motor rail is commonly used during in-hospital rehabilitation. The single-joint hybrid assistive limb (HAL-SJ) is a new therapy device. The aim of the study was to improve patients’ range of motion (ROM), mobility, and satisfaction using the active-assistive support of the HAL-SJ. Materials and methods Between 09/2017 and 10/2020, 34 patients, who underwent TKA and matched the inclusion criteria, were randomized into study (HAL-SJ) and control (CPM) group. Treatment began after drain removal and was carried out until discharge. Primary outcome parameters were raised pre- and postoperatively and included the Oxford knee score (OKS), visual analog scale (VAS), and acquired range of motion. Furthermore complications caused by the device were recorded. Results OKS increased in both groups postoperatively, but only significantly in the HAL-SJ group. Postoperative pain improved in both groups without significant differences. Flexion improvement was significant in both groups between days 3/7 and 8 weeks postoperatively. We did not encounter any complications related to HAL-SJ. Conclusions In conclusion, use of the HAL-SJ during rehabilitation in the early postoperative period after TKA was safe without disadvantages compared to the control group and seems to have advantages in terms of daily life impairment.