The Theory of Planned Behavior (TPB) serves to understand determinants that predict the intention to exercise. According to this theory, attitudes, subjective norms, and perceptions of behavioral control determine intention. This is the first theory-based tool designed to measure the determinants of exercise among women in Puerto Rico who are breast cancer survivors. Understanding the determinants will assist in planning theory based interventions. The purpose of this study was to develop a TPB-based questionnaire to assess the determinants of exercise of breast cancer survivors in Puerto Rico and to evaluate its psychometric properties. Quantitative and qualitative methods were used for questionnaire development and psychometric testing. Three independent samples were recruited for the phases of item generation, pilot testing, and evaluation of psychometric properties. An initial 97-item questionnaire was constructed. Test–retest reliability was assessed for the indirect subscales; six items were found unreliable and removed. For the direct subscales, seven items with item-to-total correlations <0.30 were removed. The final version consisted of 84 items, with Cronbach’s α ranging from 0.65 to 0.89. Construct validity was demonstrated by significant, fair-to-moderate correlations of all but one of the direct subscales and the multiplied scores of the indirect subscales of similar constructs.