Abstract Context Older adults have the greatest burden of diabetes; however, the contribution of age-related muscle loss to its development remains unclear. Objective We assessed the relationship of lean body mass with aging to incident diabetes in community-dwelling adults. Design and Setting We studied participants in the Baltimore Longitudinal Study of Aging with median follow-up of 7 years (range 1-16). Cox proportional hazard models with age as the time scale were used. Time-dependent lean body mass measures were updated at each follow-up visit available. Participants Participants included 871 men and 984 women without diabetes who had ≥ 1 assessment of body composition using dual x-ray absorptiometry. Main Outcomes Incident diabetes, defined as self-reported history and use of glucose-lowering medications; or fasting plasma glucose ≥ 126 mg/dL and 2-hour oral glucose tolerance test glucose ≥ 200 mg/dL either at the same visit or 2 consecutive visits. Results The baseline mean [standard deviation] age was 58.9 [17.3] years. Men and women with a higher percentage of total lean body mass had lower fasting and 2-hour glucose levels, and less prediabetes (all P < 0.01). Among men, comparing highest versus lowest quartiles, percentage of total lean body mass (hazard ratio [HR], 0.46; 95% confidence interval, 0.22-0.97), percentage leg lean mass (HR, 0.38; 0.15-0.96), and lean-to-fat mass ratio (HR, 0.39; 0.17-0.89) were inversely associated with incident diabetes after accounting for race and attenuated after adjustment for height and weight. Conversely, absolute total lean body mass was positively associated with incident diabetes among women, with similar trends in men. No associations were observed with muscle strength or quality. Conclusions Relatively lower lean body mass with aging is associated with incident diabetes in men and partially related to anthropometrics, but not so in women.