AIM: To evaluate the efficacy and safety of mealtime or post-meal fast-acting insulin aspart (faster aspart) vs mealtime insulin aspart (IAsp), both in combination with insulin degludec, in participants with type 1 diabetes (T1D). METHODS: This multicentre, treat-to-target trial (Clinical trial registry: NCT02500706, ClinicalTrials.gov) randomized participants to double-blind mealtime faster aspart (n = 342) or IAsp (n = 342) or open-label post-meal faster aspart (n = 341). The primary endpoint was change from baseline in HbA1c 26 weeks post randomization. All available information, regardless of treatment discontinuation, was used for evaluation of the effect. RESULTS: Non-inferiority for the change from baseline in HbA1c was confirmed for mealtime and post-meal faster aspart vs IAsp (estimated treatment difference [ETD]: 95%CI, -0.02% [-0.11; 0.07] and 0.10% [0.004; 0.19], respectively). Mealtime faster aspart was superior to IAsp for 1-hour PPG increment using a meal test (ETD, -0.90 mmol/L [-1.36; -0.45]; P < 0.001). Self-monitored 1-hour PPG increment favoured faster aspart at breakfast (ETD, -0.58 mmol/L [-0.99; -0.17]; P = 0.006) and across all meals (-0.48 mmol/L [-0.74; -0.21]; P < 0.001). Safety profiles and overall rate of severe or blood glucose-confirmed hypoglycaemia were similar between treatments, but significantly less hypoglycaemia was seen 3 to 4 hours after meals with mealtime faster aspart. CONCLUSION: Mealtime and post-meal faster aspart in conjunction with insulin degludec provided effective glycaemic control compared with IAsp, with no increased safety risk. Mealtime faster aspart provided PPG control superior to that of IAsp.