Background: Saudi Arabia has faced a considerable rise of non-communicable diseases (NCD) over the past decade. Dietary changes are essential for treatment efficacy in various NCD, but local evidence indicates rather poor treatment compliance. Knowledge about the behavioral determinants of patients can help to improve intervention adherence. The self-determination theory proposes autonomy and competence perceptions towards healthy eating to play a determining role in motivation and behavioural regulation. The aim of this study was to explore diet practice, motivation, autonomy and competence perceptions in Saudi patients with NCD.Methods: A questionnaire-based, cross-sectional study was implemented to evaluate relationships between diet habits and autonomy and competence perceptions towards healthy diet practice in patients with cardiovascular disease, diabetes and hypertension in a governmental hospital in Riyadh.Results: 269 patients >18 years participated in the study. Self-reported diet was mediocre, characterized by low fruit and vegetable intake. Hypertensive patients showed substantially poorer diet and, at the same time, higher motivational quality compared to other patient groups (p<0.05). Generally, patients demonstrated moderate motivational quality and high perceived competence, but certain sub-populations with specific commonalities strongly deviated from the norm. Competence perceptions, autonomous, as well as controlled motivation correlated with healthy diet practice (p=0.000).Conclusions: Our results indicate that both, autonomous and controlled motivation influence positive diet practice in NCD patients. We suspect a patient-group-specific exposure to health education to impact motivational quality. The influence of psychological factors on patient health behaviour is still greatly underestimated in clinical dietary interventions in KSA.