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: 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 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 pre-defined “window” of 10-14 months. Participants in the intervention group had lower HbA1c than those in the control (mean difference -0.24%; 95% Confidence Interval -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 pre-specified subgroup analysis of participants who had had diabetes for less than 7 years 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.