Background: Insulin efsitora alfa (efsitora) is a once-weekly basal insulin. This phase 3 study aimed to assess the efficacy and safety of efsitora compared with insulin degludec (degludec) in adults with type 1 diabetes., Methods: This randomised, 52-week, parallel-design, open-label, treat-to-target non-inferiority study conducted at 82 global health-care centres, randomly assigned (1:1) adults (ie, those aged ≥18 years) with type 1 diabetes glycated haemoglobin A 1c (HbA 1c ) 7·0-10·0% (53·0-85·8 mmol/mol) to efsitora (n=343) or, degludec (n=349), both in combination with insulin lispro. The primary endpoint was the change in HbA 1c from baseline to week-26 (non-inferiority margin=0·4%). The trial was registered at ClinicalTrials.gov (NCT05463744) and is completed., Findings: Between Aug 12, 2022, and May 7, 2024, of 893 participants enrolled, 692 (77%) participants were randomly assigned to once-weekly efsitora or once-daily degludec, and 623 (90%) participants completed the study. Mean HbA 1c decreased from 7·88% (62·66 mmol/mol) at baseline to 7·41% (57·5 mmol/mol) at week 26 with efsitora and from 7·94% (63·3 mmol/mol) at baseline to 7·36% (56·9 mmol/mol) at week 26 with degludec. Mean HbA 1c change from baseline to week 26 was -0·51% with efsitora and -0·56% with degludec (estimated treatment difference 0·052%, 95% CI -0·077 to 0·181; p=0·43), confirming a non-inferiority margin of 0·4% for efsitora compared with degludec. Rates of combined level 2 (<54 mg/dL [3·0 mmol/L]) or level 3 severe hypoglycaemia were higher with efsitora compared with degludec (14·03 vs 11·59 events per patient year of exposure; estimated rate ratio 1·21, 95% CI 1·04 to 1·41; p=0·016) during weeks 0-52, with the highest rates during weeks 0-12. Severe hypoglycaemia incidence was higher with efsitora (35 [10%] of 343) versus degludec (11 [3%] of 349) during weeks 0-52. Overall incidence of treatment-emergent adverse events was similar across treatment groups. One death not related to the study treatment occurred in the degludec group., Interpretation: In adults with type 1 diabetes, once-weekly efsitora showed non-inferior HbA 1c reduction compared with daily insulin degludec. Higher rates of combined level 2 or level 3 hypoglycaemia and greater incidence of severe hypoglycaemia in participants treated with efsitora compared with participants treated with degludec might suggest the need for additional evaluation of efsitora dose initiation and optimisation in people with type 1 diabetes., Funding: Eli Lilly and Company., Competing Interests: Declaration of interests RMB has received research support from, consulted, or has been on a scientific advisory board for Abbott Diabetes Care, Ascensia, Bigfoot Biomedical, CeQur, Dexcom, Eversense, Hygieia, Insulet, Eli Lilly and Company, Medtronic, Novo Nordisk, Onduo, Roche Diabetes Care, Sanofi, Tandem Diabetes Care, United Healthcare, Vertex Pharmaceutical, and Zealand Pharma. His technology research is funded in part by the National Institute of Diabetes and Digestive and Kidney Diseases and Helmsley Charitable Trust. RMB's employer, non-profit HealthPartners Institute, contracts him for his services, and no personal income from any of these activities goes directly to him. RSW has participated in multicentre clinical trials, through her institution, sponsored by Eli Lilly and Company, Insulet, Tandem, Amgen, MannKind, NovoNordisk, Wolters Kluwer, and Diasome, and has used Dexcom devices obtained at reduced cost in clinical research studies. CM serves or has served on advisory panels for Novo Nordisk, Sanofi, Eli Lilly and Company, Dexcom, Boehringer Ingelheim, Bayer, Roche, Medtronic, Imcyse, Insulet, Biomea Fusion, ActoBio Therapeutics, AstraZeneca, and Vertex. Financial compensation for these activities has been received by KU Leuven; KU Leuven has received research support for CM from Medtronic, Novo Nordisk, Sanofi, and ActoBio Therapeutics. CM serves or has served on the speaker's bureau for Novo Nordisk, Sanofi, Eli Lilly and Company, Medtronic, and Boehringer Ingelheim. Financial compensation for these activities has been received by KU Leuven. YO has received speaker's bureau fees from Eli Lilly, Novo Nordisk, and Sumitomo Pharma. AMC, MLK, MCC, and VV are employees and shareholders of Eli Lilly and Company., (Copyright © 2024 Elsevier Ltd. All rights reserved, including those for text and data mining, AI training, and similar technologies.)