Background: The Theory of Planned Behaviour (TPB) has been proposed as an appropriate model for creating a theory-driven approach to teaching medical professionalism. However, there is a lack of empirical evidence into its efficacy. This study explores if the TPB can assess UK medical doctors' professional behaviours and explores if there are differences in the TPB's efficacy depending on doctors' primary medical qualification (UK or outside). Methods: Three hundred fourteen doctors in England at 21 NHS Trusts completed a questionnaire about reflective practice, using the General Medical Council's confidentiality guidance, and raising a patient safety concern. The majority of participants were male (52%), white (68%), consultants (62%), and UK medical graduates (UKGs) (71%). Results: The TPB variables of attitudes, subjective norms, and perceived behavioural control were predictive of intention to engage in raising concerns (R 2 = 35%), reflection (R 2 = 52%), and use of confidentiality guidance (R 2 = 45%). Perceived behavioural control was the strongest predictor of intentions to raise a concern (β = 0.44), while attitude was the strongest predictor of intentions to engage in reflective practice (β = 0.61) and using confidentiality guidance (β = 0.38). The TBP constructs predicted intention for raising concerns and reflecting for both UKGs and non-UKGs (Fs ≥ 2.3; ps ≤ .023, βs ≥ 0.12). However, only perceived behaviour control was predictive of intentions to use guidance for both UKGs and non-UKGs (β = 0.24) while attitudes and norms were just predictive for UKGs (βs ≥ 0.26).Conclusions: This study demonstrates the efficacy of the TPB for three professional behaviours. The implications for medical educators are to use the variables of the TPB (attitudes, subjective norms, and perceived behavioural control) in the education of professionalism, and for medical education researchers to further our understanding by employing the TPB in more empirical studies of non-clinical behaviours.