Introduction Nonadherence to immunosuppressive (IS) therapy is associated with poor outcomes. Identifying factors predicting poor adherence is therefore essential. The primary objective of this study was to test whether parameters of a model adapted from the theory of planned behavior, and more specifically attitudes that are influenced by beliefs and satisfaction with medication, could predict adherence in solid organ transplant patients. Methods Adherence was assessed with a self-reported medication adherence scale and IS blood trough concentrations over 6 months, in four transplant units. Satisfaction and beliefs were assessed using the Treatment Satisfaction with Medicines Questionnaire (SATMED-Q) and Beliefs about Medicines Questionnaire (BMQ), respectively. Theory of planned behavior was assessed with a specific questionnaire exploring intentions, subjective norms, attitudes and perceived behavioral control. Treatment characteristics and socioeconomic data were also collected. Results One hundred and fifty-three solid organ transplant patients were enrolled, including lung (n=33), heart (n=43), liver (n=42), and kidney (n=44) patients. Satisfaction and positive beliefs about medication were higher in adherent than those in nonadherent patients. Factors independently associated with an increased risk of nonadherence were negative general beliefs about medications (odds ratio [OR]=0.89 [0.83–0.97]), living alone (OR=2.78 [1.09–7.09]), heart transplantation (OR=3.49 [1.34–9.09]), and being on everolimus (OR=5.02 [1.21–20.8]). Conclusion Negative beliefs toward medications were shown to be an independent risk factor of poor adherence. Therefore, the BMQ could be an effective, easy to implement tool, for use in everyday practice, to identify patients needing interventions to improve adherence to IS.