Background/Context: Children without siblings (singletons) have higher rates of obesity than do children with siblings (nonsingletons). Higher moderate to vigorous physical activity (PA) and less sedentary behavior (SB) are associated with lower childhood obesity. Purpose: To examine the difference in PA and SB between singleton and nonsingleton children. Methods: Mothers of children ages 5.0–7.9 years old who were singletons or nonsingletons with a sibling between the ages of 2.0 and 4.9 years old were recruited. Height, weight, and waist circumference of the 5.0- to 7.9-year-old children were measured, and age and sex percentiles were calculated. Accelerometry measured SB and PA, including light PA, moderate to vigorous PA, and counts per minute. Results: Fifty-six mother–child dyads (23 singletons and 33 nonsingletons) with an average child age of 5.7 (0.7) years participated. More singletons were classified as overweight or obese than were nonsingletons (49% vs 17%, P = .04). In adjusted linear models, singletons had less light PA per day (β = −38.1, SE = 19.2, P = .001) and more SB (β = 38.0, SE = 16.5, P = .02) than did nonsingletons, with no difference in moderate to vigorous PA or counts per minute. Conclusion: In this sample, singletons had higher obesity and lower light PA than did nonsingleton children. Investigation into differences in singleton/nonsingleton families, including family health behaviors, may help assess sibling influence in early behavior development.