The objectives of this study were to test the associations between physical self-discrepancies (actual:ideal and actual:ought) and physical activity behavior, and to examine whether motivational regulations mediate these associations using self-discrepancy (Higgins, 1987) and organismic integration (Deci & Ryan, 1985) theories as guiding frameworks. Young women (N = 205; M age = 18.87 years, SD = 1.83) completed self-report questionnaires. Main analyses involved path analysis using a polynomial regression approach, estimation of direct and indirect effects, and evaluation of response surface values. Agreement between actual and ideal (or ought) physical self-perceptions was related to physical activity both directly and indirectly as mediated by the motivational regulations (R 2 = .24–.30). Specifically, when actual and ideal self-perceptions scores were similar, physical activity levels increased as actual and ideal scores increased. Furthermore, physical activity levels were lower when the discrepancy was such that ideal or ought self were higher than actual self. These findings provide support for integrating self-discrepancy and organismic integration theories to advance research in this area.