PurposeAddressing grade 2 and 3 medial‐sided instabilities during anterior cruciate ligament (ACL) reconstruction is crucial to reduce the risk of ACL graft failure. This study introduced a minimally invasive, medial collateral ligament (MCL) augmentation technique using a peroneus longus split tendon autograft, which was fixed to the femoral deep MCL insertion and tibial superficial MCL insertion.MethodsThis prospective, longitudinal, single‐centre case series included patients who underwent MCL augmentation concurrent with primary or revision ACL reconstruction due to anteromedial instability. Preoperatively and at 1‐year follow‐up, clinical examinations, such as rolimeter test of anterior tibial translation and medial instability, knee (International Knee Documentation Committee subjective knee form [IKDC], Lsyholm, Knee Injury and Osteoarthritis Outcome Score [KOOS]) and foot function scores (American Orthopaedic Foot and Ankle Society score [AOFAS]) and complications, were analyzed.ResultsThirty‐one patients with a mean follow‐up of 13.5 ± 2.6 months and a mean age of 27.8 ± 9.6 years were included. The side‐to‐side difference for anterior tibial translation significantly improved from preoperative to 1‐year follow‐up with an ACL reconstruction failure rate of 6.5%. No patient retained a grade 2 or 3 medial instability on valgus stress testing with 30° flexion. Significant improvements from preoperative to 1‐year postoperative follow‐up were observed in knee function scores: IKDC (48.9 ± 26.9– 71.3 ± 11.5, p < 0.001) and Lysholm (59.9 ± 28.5–80.5 ± 11.2, p = 0.002) as well as KOOS pain, ADL, sport and QoL, each reaching the respective minimal clinically important difference values. The foot function score AOFAS showed no significant impairment (100 ± 0–99.3 ± 2.5, p = 0.250). Complications included cyclops lesions of ACL reconstruction in three patients.ConclusionAt 1‐year follow‐up, MCL augmentation using a peroneus longus split tendon autograft for patients simultaneously undergoing ACL reconstruction satisfactorily restores knee stability, has a low ACL reconstruction failure rate and does not significantly impair foot function.Level of EvidenceLevel IV therapeutic study; case series.