IntroductionDiabetes mellitus (DM) and its severe complications contribute significantly to disability and, hence, burden of disease. Poor mental health, a frequent DM consequence, may hinder successful diabetes control in low-income and middle-income countries (LMICs). Evidence suggests social support as a cost-effective tool to improve diabetes self-management, behaviour and mental health. However, its real-world application in LMICs has rarely been tested. We aim to investigate the effect of a social support intervention on disease control, mental health and health-related quality of life in people with diabetes from Côte d’Ivoire (SoDDiCo) through a randomised controlled trial.Methods and analysisThe trial will take place in the Centre Antidiabétique d’Abidjan, Institut National de Santé Publique, Abidjan, Côte d’Ivoire. We will prospectively randomise up to 1500 people with newly diagnosed diabetes into two parallel arms: intervention (routine care+family supporter accompanying clinical management) and control (routine care), using gender-stratified blocked randomisation with random block sizes of 10, 16, 20 and 24. Participants will undergo baseline, 3-month and 12-month postrandomisation assessments. The primary study outcome will be glycated haemoglobin (HbA1c). Secondary outcomes will include glycaemic control (HbA1c<7.0%), presence at follow-up visits, mental health and quality of life scores. Using intention-to-treat framework, we will assess the impact of the family support intervention on these endpoints over the course of the 1-year follow-up. Effect modification by baseline social capital will be assessed.Ethics and disseminationThe SoDDiCo trial was approved by the Ethikkommission Nordwest- und Zentralschweiz (ref: AO_2021-00041; approved: 12 July 2021) and by Comité National d'Éthique des Sciences de la Vie et de la Santé (ref: 049-22/MSHPCMU/CNESVS-kp; approved: 20 April 2022). The randomised intervention trial will follow good clinical practice guidelines. All results will be made available to the public through abstracts at conferences as well as through peer-reviewed articles. International guidelines for authorship will be respected.Trial registration numberISRCTN10901121, ISRCTN registry.