BackgroundMetabolic syndrome (MetS) is a global health problem. Physical activity (PA) is a known modifiable risk factor for MetS and individual MetS components. However, the role of PA could differ between sub-populations due to differences in the variability of PA and other MetS risk factors. To examine these differences, multi-country studies with standardized outcome measurement methods across cohorts are needed. MethodsCross-sectional PA levels (total and domain specific) in healthy middle-aged (44–56 years) men in the Risk Factor Assessment among Japanese and U.S. Men in the Post-World War II Birth Cohort (ERA-JUMP) Study (n = 730; American: n = 417; Japanese: n = 313; from population-representative samples in Pittsburgh, Pennsylvania, USA, and Kusatsu, Shiga, Japan) were compared. The relationships between PA levels and MetS (overall and specific components) in/across the American and Japanese sub-cohorts (adjusting for age, smoking, and alcohol consumption) were also assessed using the same instruments (pedometer and validated questionnaire) to measure PA in both cohorts. ResultsA total of 510 individuals provided complete data on PA (American: n = 265; Japanese: n = 245). The American cohort had significantly lower mean ± SD steps/day (7878 ± 3399 steps/day) vs. the Japanese cohort (9055 ± 3797 steps/day) (p < 0.001) but had significantly higher self-reported moderate-vigorous leisure PA (American: 15.9 (7.4–30.3) metabolic task equivalent hours per week (MET-h/week) vs. Japanese: 4.0 (0–11.3) MET-h/week, p < 0.0001). In both sub-cohorts, each 1000 steps/day increase was associated with lower odds of having MetS (American: OR = 0.90, 95%CI: 0.83–0.98; Japanese: OR = 0.87, 95%CI: 0.79–0.95) and the individual MetS component of high waist circumference (American: OR = 0.86, 95%CI: 0.79–0.94; Japanese: OR = 0.87, 95%CI: 0.80–0.95). In the American cohort only, higher self-reported leisure PA (Met-h/week) was associated with lower odds of MetS and high waist circumference (OR = 0.98, 95%CI: 0.97–0.99 for MetS and waist circumference, respectively). ConclusionHigher total step counts/day had an important protective effect on MetS prevalence in both the Japanese and American cohorts, despite differences in PA levels and other MetS risk factors. The effect of steps/day (across all intensity levels) was much greater than domain-specific moderate-vigorous PA captured by questionnaire, suggesting the need for measurement tools that can best capture total movement when examining the effects of PA on MetS development.