ABSTRACT Objective: to elaborate and validate a Nursing Theory for Patients' Compliance with the Treatments of Arterial Hypertension and Diabetes Mellitus. Method: this is a theoretical and methodological study that followed four stages, namely: development of the concepts; development and validation of proposals; structuring and contextualization of the theory; and content validation. The validity process took place in two rounds, from July to September 2018, with the participation of 7 expert judges in the field of Nursing theories and experience in monitoring hypertensive and diabetic patients. Results: the Nursing Theory for Patient's Compliance with the treatments of Arterial Hypertension and Diabetes Mellitus is composed of 18 concepts and has adherence as a central element. The conceptual structure shows that the studied phenomena are inserted in open systems in constant interaction, in which changes in any of those involved can reflect on the others, directly impacting on patients' compliance the treatment and control of the aforementioned chronic diseases. The Content Validity Index of the first round was 0.79, and that of the second 0.97, showing a positive evolution of the altered proposals in compliance with the judges' recommendations. Content validation showed adequate internal consistency in the two rounds, with a Cronbach's alpha value of 0.79 in the first, and of 0.80 in the second. Conclusion: the elaboration and validation of the Theory proved to be consistent with the proposal of clinical Nursing care for people with Hypertension and Diabetes, whose focus is adherence to their treatments.