Objectives: Self-regulation for physical activity is considered as one of the most effective factors in promoting physical activity. However, there is no reliable and valid scale to measure it in Japanese. The purpose of this study was to investigate the internal consistency, convergent validity, and structural validity of the newly developed Japanese version of the 12-item Physical Activity Self-Regulation scale (PASR-12) among Japanese workers. Methods: A cross-sectional Internet-based survey recruiting 516 Japanese workers was conducted in Japan. The PASR-12 was translated according to the International Society of Pharmacoeconomics and Outcomes Research (ISPOR) task force guidelines. Physical activity and self-efficacy for physical activity were measured as comparisons for convergent validity. We calculated Cronbach's alphas, and conducted correlational analyses and confirmatory factor analysis (CFA). Results: Of 516 workers, 485 workers were eligible for all analyses. Cronbach's alpha for the scale scores ranged from 0.79 to 0.95. The scores of the total and 6 factor scales of the Japanese version of the PASR-12 had small-to-moderate positive correlations with the total physical activity and self-efficacy. Moreover, the 6-factor hypothesized model demonstrated excellent fit (χ2 (39) = 100.74, CFI = 0.973, RMSEA = 0.057). Conclusions: The Japanese version of the PASR-12 showed good reliability and factor-based and construct validity. Therefore, this scale could be applied to assess self-regulation for physical activity among Japanese workers.