Background The benefit of initiation of insulin pump therapy (continuous subcutaneous insulin infusion; CSII) in patients with type 1 diabetes using continuous glucose monitoring (CGM) has not been studied. We aimed to assess glycaemic outcomes when switching from multiple daily injections (MDI) to CSII in adults with type 1 diabetes using CGM. Methods In this multicentre, randomised controlled trial, 75 adults with type 1 diabetes in the CGM group of the DIAMOND trial were randomly assigned via the study website using a computer-generated sequence to continue MDI or switch to CSII, with continuation of CGM, for 28 weeks. The primary outcome was CGM-measured time in the glucose concentration range of 70–180 mg/dL (3·9–10·0 mmol/L). This study is registered with ClinicalTrials.gov, number NCT02282397. Findings Between April 14, 2015, and May 5, 2016, 37 participants were randomly assigned to the CGM plus CSII group and 38 participants were randomly assigned to the CGM plus MDI group. The study was completed by 36 (97%) of 37 participants in the CGM plus CSII group and 35 (92%) of 38 participants in the CGM plus MDI group. Mean CGM use was 6·7 days per week (SD 0·8) in the CGM plus CSII group and 6·9 days per week (0·3) in the CGM plus MDI group (p=0·86). No participants in the CGM plus CSII group who completed the trial discontinued CSII. Over the follow-up period, mean time in the glucose concentration range of 70–180 mg/dL (3·9–10·0 mmol/L) was 791 min per day (SD 157) in the CGM plus CSII group and 741 min per day (225) in the CGM plus MDI group (adjusted mean treatment group difference: 83 min, 95% CI 17–149; p=0·01). Participants in the CGM plus CSII group had a greater reduction in CGM-measured mean glucose (p=0·005) and hyperglycaemia (on four metrics: p=0·007 for >180 mg/dL [>10·0 mmol/L], p=0·02 for >250 mg/dL [>13·9 mmol/L], p=0·04 for >300 mg/dL [>16·6 mmol/L], and p=0·02 for the area under the curve for 180 mg/dL [10·0 mmol/L]), but also an increase in CGM-measured hypoglycaemia (p=0·0001 for <70 mg/dL [<3·9 mmol/L], p=0·0002 for <60 mg/dL [<3·3 mmol/L], p=0·0009 for <50 mg/dL [<2·8 mmol/L], p=0·0002 for the area over the curve for 70 mg/dL [3·9 mmol/L]). Mean HbA1c change from baseline to 28 weeks was 0·3% (SD 0·9; 3·3 mmol/mol [SD 9·8]) in the CGM plus CSII group and 0·1% (0·4; 1·1 mmol/mol [4·4]) in the CGM plus MDI group (p=0·32). Severe hypoglycaemia occurred in one participant in the CGM plus MDI group, and diabetic ketoacidosis and severe hyperglycaemia occurred in one participant each in the CGM plus CSII group. Interpretation Our findings show that glycaemic control measured by time in the glucose range of 70–180 mg/dL (3·9–10·0 mmol/L) is improved by initiation of CSII in adults with type 1 diabetes. However, biochemical hypoglycaemia also was increased in the study, which will be important to consider when incorporating these results into clinical practice. Funding Dexcom.