Background: The purpose of this study was to determine the association of regular leisure-time physical activity with body mass index and obesity risk in Taiwanese young adults. Methods: A total of 10,802 young adults (18-44 years) were enrolled into this national telephone survey. The questionnaire data of this survey includes socio-demographic characteristics, zip code of residence, leisure-time physical activity (LTPA) behaviors, self-reported health status, and self-evaluations (including height, body weight, and body mass index [BMI]). Results: When using non-regular LTPA as the baseline, participants in regular LTPA group exhibited the lower risks of overweight and underweight (OR, 0.837; 95% CI 0.738–0.948, OR, 0.732; 95% CI 0.611-0.876). But there was no significant relationship between the regular LTPA and obesity risk when using non-regular LTPA as the baseline after adjusting for potential confounders. Conclusions: The study results revealed that regular LTPA effectively lowered the risks of underweight and overweight. However, for people with obesity, regular LTPA was unable to significantly decrease their obesity risk.