Aim: To investigate the efficacy of adding the glucagon-like peptide-1 receptor agonist liraglutide to continuous subcutaneous insulin infusion (CSII) in overweight or obese persons with type 1 diabetes and non-optimal glycaemic control. Materials and methods: A 26-week, randomized, double-blind, placebo-controlled trial including 44 overweight or obese adults with type 1 diabetes randomized 1:1 to liraglutide 1.8 mg once daily (QD) or placebo added to CSII treatment. The primary endpoint was change in haemoglobin A1c (HbA1c). Secondary endpoints included change in insulin dose, CSII settings, glycaemic variability, body weight and patient-reported outcome measures. Finally, adverse effects including hypoglycaemic events were registered. Results: HbA1c was reduced by 5 mmol/mol (0.5%) from a baseline of 66 mmol/mol (8.2%) in patients treated with liraglutide compared with a non-significant change of +2.3 mmol/mol (0.2%) from a baseline of 66 mmol/mol (8.1%) in patients treated with placebo (between-group difference 7 mmol/mol [0.7%], P < 0.001). Liraglutide reduced total insulin dose by 8 units/day or 16% of total insulin dose (P = 0.008). Mean body weight was reduced by 6.3 kg (P < 0.001) compared with placebo. Concomitantly, time spent in glycaemic target range 4–10 mmol/L (71–180 mg/dL) increased while the risk of hypoglycaemia did not differ between groups at the end of treatment. Conclusion: Liraglutide treatment reduced HbA1c, total daily insulin dose and body weight without increasing the risk of hypoglycaemia in CSII-treated patients with type 1 diabetes and insufficient glycaemic control. Liraglutide may be considered a potential add-on therapy to insulin in this subgroup of patients.