We evaluated the effectiveness of a low-carbohydrate digitally-supported weight loss programme for glycaemic control for people with type 2 diabetes (T2D) compared with usual primary care in a 12 month RCT. We individually randomised 115 people with T2D and BMI ≥27 kg/m2 recruited from 19 general practices in England, to receive either a 12-week low-carbohydrate programme with digital support, or usual care. There was no between-group difference in HbA1c change from baseline to 3 or 12 months (primary outcome; estimated mean difference (95% CI) –0.7 mmol/mol (–5.0 to 3.6), and –1.5 (–5.7 to 2.8), p = 0.80). Greater mean weight loss in the intervention group at 3 months (2.6 kg (0.6 to 4.6)) was not sustained by 12 months (–0.4 kg (–2.3 to 1.6)). While this digitally-delivered intervention was acceptable to patients, there was no evidence of a meaningful impact of the intervention on glycaemia or other cardiovascular risk factors beyond that achieved with usual care. This trial was prospectively registered at clinicaltrials.gov, NCT04916314, in June 2021.