Background: The capacity of physical therapists to reason effectively in patient management is essential to maximizing outcomes. Although conceptual frameworks of clinical reasoning exist, their theoretical foundations are insufficiently validated to establish those factors that are paramount in guiding physical therapists' clinical reasoning. Studies on how physical therapists clinically reason constitute important means of identifying constructs of such reasoning. Objective: This systematic review aimed to synthesize and interpret the findings of qualitative studies designed to examine factors that are inherent in physical therapists' clinical reasoning with respect to their knowledge, experiences, and practices. Methods: Searches of studies were carried out in four databases, gray literature, and reference lists. Two reviewers independently assessed methodological quality of the studies using the Critical Appraisal Skills Program (CASP) and performed the analysis: extraction and comparative appraisal of findings, identification of themes, reciprocal translation synthesis, and identification of categories and subcategories. Results: Ten studies were included. Four themes of factors influencing physical therapists' clinical reasoning emerged, namely, Physical therapist as a source, Patient as a source, Elements of the reasoning process, and Context. Conclusions: The identified themes validated some constructs underlying existing clinical reasoning frameworks. Most influencing factors were related to the physical therapist, which highlights opportunities to improve effective reasoning at this level. The notion that this process is recurrent, multifaceted, and contextual lends itself to changing in accordance with the needs of the patient, consistent with a biopsychosocial perspective. How clinicians weigh biomedical and psychosocial elements in their reasoning however warrants further study.