Objectives: To investigate the relationship between sleep duration and obesity in older Australian adults. Method: Self-reported health data were collected through the 45 and Up Study. Multinomial logistic regression models were used to test the relationship between sleep duration and body mass index, controlling for a range of demographic and health-related variables. Results: The sample included 45,325 Australian adults aged 55 to 95 years. The regression models demonstrated that short and long sleep were associated with obesity in 55- to 64-year-olds but not in those aged 65 years and above. Discussion: The present results suggest that the relationships between sleep duration and obesity previously reported in young and middle-aged adults are not evident in older adults. The absence of these relationships could reflect a combination of age-related factors that impact on sleep and body composition.