IMPORTANCE Observational studies suggest that vitamin D supplementation is associated with benefits for knee osteoarthritis, but current trial evidence is contradictory. OBJECTIVE To compare the effects of vitaminDsupplementation vs placebo on knee pain and knee cartilage volume in patients with symptomatic knee osteoarthritis and lowvitaminDlevels. DESIGN, SETTING, AND PARTICIPANTS A multicenter randomized, double-blind, placebo-controlled clinical trial in Tasmania and Victoria, Australia. Participants with symptomatic knee osteoarthritis and low 25-hydroxyvitamin D (12.5-60 nmol/L) were enrolled from June 2010 to December 2011. The trial was completed in December 2013. INTERVENTIONS Participants were randomly assigned to receive monthly treatment with oral vitamin D3 (50 000 IU; n = 209) or an identical placebo (n = 204) for 2 years. MAIN OUTCOMES AND MEASURES Primary outcomeswere change in tibial cartilage volume (assessed usingmagnetic resonance imaging [MRI]) and change in theWestern Ontario and McMaster Universities Arthritis Index (WOMAC) pain score (0 [no pain] to 500 [worst pain]) from baseline to month 24. Secondary outcomes were cartilage defects and bone marrow lesions (assessed using MRI). RESULTS Of 413 enrolled participants (mean age, 63.2 years; 50% women), 340 (82.3%) completed the study. The level of 25-hydroxyvitamin D increased more in the vitamin D group (40.6 nmol/L) than in the placebo group (6.7 nmol/L) (P <.001) over 2 years. There were no significant differences in annual change of tibial cartilage volume orWOMAC pain score. There were no significant differences in change of tibiofemoral cartilage defects or change in tibiofemoral bone marrow lesions. Adverse events (≥1 per patient) occurred in 56 participants in the vitamin D group and in 37 participants in the placebo group (P =.04). CONCLUSIONS AND RELEVANCE Among patients with symptomatic knee osteoarthritis and low serum 25-hydroxyvitamin D levels, vitamin D supplementation, compared with placebo, did not result in significant differences in change in MRI-measured tibial cartilage volume or WOMAC knee pain score over 2 years. These findings do not support the use of vitamin D supplementation for preventing tibial cartilage loss or improvingWOMAC knee pain in patients with knee osteoarthritis.