INTRODUCTION: Type 2 diabetes mellitus is a major global challenge, including for Thai policy-makers, as an estimated 4 million people in Thailand (population 68 million) have this condition. Premature death and disability due to diabetes are primarily due to complications which can be prevented by good risk factor control. Diabetes Self-Management Education (DSME) programmes provide patients with diabetes with the necessary knowledge and skills to effectively manage their disease. Currently, a trial is being conducted in Thailand to evaluate the effectiveness, defined as HbA1c<7 at 12 months after enrolment, of a culturally tailored DSME in Thailand. A process evaluation can provide further interpretation of the results from complex interventions as well as insight into the success of applying the programme into a broader context. METHODS AND ANALYSIS: The aim of the process evaluation is to understand how and why the intervention was effective or ineffective and to identify contextually relevant strategies for future successful implementation. For the process evaluation, the design will be a mixed-method study collecting data from nurse providers, and village health volunteers (community health workers) as well as patients. This will be conducted using observations, interviews and focus groups from the three purposively selected groups at the beginning and end of trial. Quantitative data will be collected through surveys conducted at the beginning, during 6-month follow-up, and at the end of trial. The mixed-methods analysis will be triangulated to assess differences and similarities across the various data sources. The overall effectiveness of the intervention will be examined using multilevel analysis of repeated measures. ETHICS AND DISSEMINATION: Study approved by the Chiang Mai University Research Ethics Committee (326/2018) and the London School of Hygiene & Tropical Medicine (16113/RR/12850). Results will be published in open access, peer-reviewed scientific journals. TRIAL REGISTRATION NUMBER: NCT03938233.