OBJECTIVE: To determine the effectiveness of a web-based self-management programme for people with type 2 diabetes in improving glycaemic control and reducing diabetes-related distress. METHODS AND DESIGN: Individually randomised two-arm controlled trial. SETTING: 21 general practices in England. PARTICIPANTS: Adults aged 18 or over with a diagnosis of type 2 diabetes registered with participating general practices. INTERVENTION AND COMPARATOR: Usual care plus either Healthy Living for People with Diabetes (HeLP-Diabetes), an interactive, theoretically informed, web-based self-management programme or a simple, text-based website containing basic information only. OUTCOMES AND DATA COLLECTION: Joint primary outcomes were glycated haemoglobin (HbA1c) and diabetes-related distress, measured by the Problem Areas in Diabetes (PAID) scale, collected at 3 and 12 months after randomisation, with 12 months the primary outcome point. Research nurses, blind to allocation collected clinical data; participants completed self-report questionnaires online. ANALYSIS: The analysis compared groups as randomised (intention to treat) using a linear mixed effects model, adjusted for baseline data with multiple imputation of missing values. RESULTS: Of the 374 participants randomised between September 2013 and December 2014, 185 were allocated to the intervention and 189 to the control. Final (12 month) follow-up data for HbA1c were available for 318 (85%) and for PAID 337 (90%) of participants. Of these, 291 (78%) and 321 (86%) responses were recorded within the predefined window of 10-14 months. Participants in the intervention group had lower HbA1c than those in the control (mean difference -0.24%; 95% CI -0.44 to -0.049; p=0.014). There was no significant overall difference between groups in the mean PAID score (p=0.21), but prespecified subgroup analysis of participants who had been more recently diagnosed with diabetes showed a beneficial impact of the intervention in this group (p = 0.004). There were no reported harms. CONCLUSIONS: Access to HeLP-Diabetes improved glycaemic control over 12 months. TRIAL REGISTRATION NUMBER: ISRCTN02123133.