Background Physical therapists play an important role in the promotion of physical activity (PA) and the effectiveness of PA interventions. However, little is known about the extent to which they implement PA interventions following the intervention protocol and about the factors influencing their implementation behaviors. Objective The study objective was to investigate physical therapists' implementation fidelity regarding PA interventions, including completeness and quality of delivery, and influencing factors with a Theoretical Domains Framework–based questionnaire. Design The study was based on a cross-sectional design. Methods A total of 268 physical therapists completed the Determinants of Implementation Behavior Questionnaire. Questions about completeness and quality of delivery were based on components and tasks of PA interventions as described by the Royal Dutch Society for Physical Therapy. Multilevel regression analyses were used to identify factors associated with completeness and quality of delivery. Results High implementation fidelity was found for the physical therapists, with higher scores for completeness of delivery than for quality of delivery. Physical therapists' knowledge, skills, beliefs about capabilities and consequences, positive emotions, behavioral regulation, and the automaticity of PA intervention delivery were the most important predictors of implementation fidelity. Together, the Theoretical Domains Framework accounted for 23% of the variance in both total completeness and total quality scores. Limitations The cross-sectional design precluded the determination of causal relationships. Also, the use of a self-report measure to assess implementation fidelity could have led to socially desirable responses, possibly resulting in more favorable ratings for completeness and quality. Conclusions This study enhances the understanding of how physical therapists implement PA interventions and which factors influence their behaviors. Knowledge about these factors may assist in the ...