There is increasing evidence for the effect of obesity on knee osteoarthritis (OA), although the association between obesity, particularly body composition, and knee osteoarthritis, using magnetic resonance imaging (MRI) to examine knee structure, has not been examined. We systematically evaluated the evidence for the relationship between obesity and knee cartilage assessed by MRI. We performed an electronic search of MEDLINE and EMBASE up to December 2012. Included studies investigated the association between obesity and the development and/or progression of knee cartilage changes using MRI. The studies were ranked according to their methodological score and best-evidence synthesis was performed to summarize the results Twenty-two studies were identified for inclusion, of which 7 were cross-sectional, 13 were longitudinal and 2 had both cross-sectional and longitudinal components. Seven cross-sectional and eight longitudinal studies were of high quality. Best-evidence synthesis showed consistent, yet limited evidence for a detrimental effect of body mass index (BMI) and fat mass on knee cartilage. This review identified a consistent detrimental effect of obesity, particularly related to elevated BMI and fat mass on cartilage defects. The strength of evidence was limited by the paucity of high-quality cohort studies examining this question. By further examining the mechanisms for these different effects, new strategies can be developed to prevent and treat knee OA.