Purpose: Peripheral nerve blocks are an important part of postoperative analgesia for the extremities. Previously, we reported that a single shot of 0.375% ropivacaine (20 mL) via ultrasound-guided popliteal sciatic and saphenous nerve blocks provided satisfactory analgesia after foot and ankle surgery; however, toe and ankle weakness in the early postoperative period became a concern for patients. Our preliminary data indicate that 0.25% ropivacaine may be effective for postoperative analgesia. Hence, we hypothesized that the analgesic effect of 0.25% ropivacaine would be noninferior to that of 0.375% ropivacaine at the same volume and would reduce the degree of weakness. Patients and Methods: In this randomized, double-blind, self-paired, noninferiority trial, 31 patients who were scheduled for similar, elective, bilateral foot and ankle surgeries under general anesthesia combined with popliteal sciatic and saphenous nerve blocks were enrolled. Each patient was randomly assigned to receive 0.25% ropivacaine on one side and 0.375% ropivacaine on the other side. The primary outcome was the duration of analgesia, which was defined as the time from the end of the nerve blocks until the first sensation of pain in the surgical area, as indicated by a patient-reported visual analog scale (VAS) score >= 1. The secondary outcomes included static VAS scores, motor and sensory block grades, patient satisfaction scores, and the incidence of adverse effects. Results: The mean duration of analgesia was 31.7 +/- 8.3 h for 0.25% ropivacaine, and 31.9 +/- 8.5 h for 0.375% ropivacaine (duration difference, - 0.16; 95% CI, - 1.5 to 1.2; P = 0.812). Compared with 0.375% ropivacaine, 0.25% ropivacaine resulted in a lower incidence of motor block at 0, 2, 6 and 12 hours postoperatively (P < 0.05). No differences in static VAS, sensory block or patient satisfaction scores were observed between the two concentrations within 48 hours postoperatively. Furthermore, no nerve block-related adverse events were reported. Conclusion: The results revealed that 0.25% ropivacaine is not inferior to 0.375% ropivacaine in terms of the analgesic duration of popliteal sciatic and saphenous nerve blocks for bilateral foot and ankle surgery. Moreover, 0.25% ropivacaine reduced the incidence of motor block. Therefore, we recommend 0.25% ropivacaine for postoperative analgesia for foot and ankle surgery.