Background: The management of chronic anterior cruciate ligament (ACL) deficient knees with concurrent varus osteoarthritis with high pivoting poses a significant challenge. This prospective pilot study investigates the clinical, functional, and radiographic outcomes of a one-stage surgical approach combining ACL reconstruction with open-wedge high tibial osteotomy (HTO). Methods: We enrolled 16 patients with chronic ACL-deficient knees and medial compartment osteoarthritis and high pivoting. All patients underwent open-wedge HTO with lateral extra-articular augmentation. We assessed clinical and functional outcomes using the Lysholm and International Knee Documentation Committee (IKDC) scores, as well as the Lachman, anterior drawer, and pivot-shift tests. Radiographic parameters, including the mechanical tibiofemoral angle (TFA), proximal medial tibial angle (PMTA), and posterior tibial slope (PTS), were measured preoperatively and at final follow-up. Results: The combined procedure yielded significant improvements in all measured outcomes. The mean Lysholm and IKDC scores increased from 47.25±3.23 and 41.5±3.25preoperatively to 66.0±5.32 and 67.5±7.03 postoperatively, respectively (p<0.001). All patients showed a reduction in pivot-shift grade, with a conversion of the mechanical axis from a median varus of −8∘ to a mild valgus of 3∘. The mean PMTA was successfully corrected from 78.88 to 88.85 ∘. Notably, the mean posterior tibial slope was reduced from 10.50∘ to 8.56∘. The complication rate was low, with no major complications such as nonunion or graft failure. Conclusion: The one-stage surgical procedure of open-wedge HTO with lateral extraarticular augmentation in chronic ACL-deficient varus knees with early osteoarthritis provides significant improvements in function and stability while reliably correcting coronal and sagittal alignment. This approach offers a safe and effective strategy for joint preservation.