Self-management is deemed the cornerstone in overall diabetes management. Web-based self-management interventions have potential to support adults with type 2 diabetes (T2DM) in managing their disease. Owing to somewhat ambiguous results of such interventions, interventions should be theory-based and incorporate well-defined counselling methods and techniques for behavioural change. This study is designed to assess the effectiveness of a theory-driven web-based Guided Self-Determination (GSD) intervention among adults with T2DM in general practice to improve diabetes self-management behaviours and glycosylated haemoglobin (HbA1c). Open Access Article