8 randomized controllled trials (3077 patients) that compared 25-hydroxyl vitamin D against placebo in patients with knee osteoarthritis were included in this systematic review and meta-analysis. Outcomes of interest included changes in tibial cartilage volume, pain on a Visual Analog Scale (VAS), synovial fluid volume, joint space width, bone marrow lesions, and the Western Ontario and McMaster Universities Arthritis Index (WOMAC) scores. A subgroup analysis of vitamin D dosage was also conducted. The results showed that tibial cartilage volume, VAS pain scores, and WOMAC function favored the vitamin D group. In the subgroup analysis of ≥ or < 2000UI.day of vitamin D, WOMAC pain favored vitamin D for ≥2000 IU/day. Vitamin D <2000IU/day for WOMAC pain was comparable between the two groups. Alternatively, synovial fluid volume favored the placebo group in the subgroup analysis of vitamin D ≥ 2000IU/day.