In recent decades, early rehabilitation after Achilles tendon rupture (ATR) repair has been proposed. The aim of this prospective cohort study was to compare different immobilisation durations in order to determine the optimal duration after open surgery for ATR repair. This study included 1088 patients (mean age, 34.9 ± 5.9 years) who underwent open surgery for acute ATR repair. The patients were categorised into four groups (A, B, C, and D) according to postoperative immobilisation durations of 0, 2, 4, and 6 weeks, respectively. All patients received the same suture technique and a similar rehabilitation protocol after brace removal,; they were clinically examined at 2, 4, 6, 8, 10, 12, 14, and 16 weeks postoperatively, with a final follow-up at a mean of 19.0 months. The primary outcome was the recovery time for the one-leg heel-rise height (OHRH). Secondary outcomes included the time required to return to light exercise (LE) and the recovery times for the range of motion (ROM). Data regarding the surgical duration, complications, the visual analogue scale (VAS) score for pain, the Achilles tendon Total Rupture Score (ATRS), and the American Orthopaedic Foot and Ankle Society (AOFAS) Ankle-Hindfoot Scale score were also collected. The recovery times for OHRH, LE, and ROM were significantly shorter in groups A and B than in groups C and D (P < 0.001). The VAS scores decreased over time, reaching 0 in all groups by 10 weeks. The mean scores in groups A and B were higher than those in the other groups at 2 and 4 weeks (P < 0.001), whereas the opposite was true at 8 weeks (P < 0.001). ATRS and the AOFAS Ankle-Hindfoot scale score increased across all groups over time, showing significant between-group differences from weeks 6 to 16 (P < 0.001) and weeks 6 to 12 (P < 0.001). The mean scores were better in groups A and B than in groups C and D. Thirty-eight complications (3.5%) were observed, including 20 re-ruptures and 18 superficial infections. All complications were resolved at the last follow-up, with no significant between-group differences. Immobilisation for 2 weeks after open surgery for ATR repair may be the optimal strategy for early rehabilitation with relatively minimal pain and other complications. ClinicalTrials.gov (NCT04663542).