Six randomized controlled trials with eight mesenchymal stem cell (MSC) treatment arms and 300 patients with knee osteoarthritis were reviewed. Trials compared a single intra-articular MSC injection (derived from adipose tissue, bone marrow, or umbilical cord; allogeneic) with saline, hyaluronic acid, or placebo, with uniform reporting of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scores at baseline and 12 months. The primary outcome was change in WOMAC total score at 12 months, pooled using a random-effects model. Subgroup analysis and meta-regression evaluated dose effects, dichotomized at ≤25 million vs >25 million cells. Overall, the analysis demonstrated a moderate-to-large improvement in WOMAC scores favoring MSC therapy. Importantly, doses ≤25 million cells were associated with statistically significant benefit, while higher doses did not confer additional improvement. These findings suggest that lower, dose-efficient MSC regimens achieve meaningful clinical benefit at one year.