Background: Knee osteoarthritis is a progressive disease that may require management for years before knee arthroplasty can be considered. Previously reported musculoskeletal models suggest that rebalancing the strength of the triceps surae muscles can reduce the joint loads. Methods: A single patient diagnosed with mild/moderate medial left knee osteoarthritis was treated with botulinum toxin injections in the gastrocnemius muscle of the calf, based on the hypothesis that this would rebalance the triceps surae load distribution and reduce tibiofemoral joint loads. Tests were performed before and 4 weeks after injection to record functional clinical scores and to obtain lower limb joint kinematic and kinetic data of walking, which were subsequently analyzed with a musculoskeletal simulation model. Findings: The patient experienced a clinically relevant improvement in self-reported pain levels in activities-of-daily-living, stair climbing, 6 minutes' treadmill test, range-of-motion, and in the functional knee questionnaire, KOOS. No improvement was seen when performing lunges. The musculoskeletal simulations showed the expected shift in loads between the muscles, reduced knee loads, and improvement of the load symmetry between the legs. Interpretation: The case corroborates the hypothesis, and this suggests further tests by randomized controlled trials. If confirmed, this simple and reversible medical intervention can improve the management of early-stage knee osteoarthritis.