This study aimed to confirm the efficacy and safety of mealtime and post-meal fast-acting insulin aspart versus insulin aspart, both with basal insulin degludec, in Japanese patients with type 1 diabetes. This was a subgroup analysis of onset 8, a randomized multicenter, treat-to-target trial of mealtime fast-acting insulin aspart (subgroup n = 73), mealtime insulin aspart (n = 83), or open-label post-meal fast-acting insulin aspart (n = 89), all for 26 weeks. Change from baseline in HbA1c was considered the primary endpoint. After 26 weeks, the estimated treatment difference (ETD, 95% CI) for change from baseline in HbA1c between mealtime fast-acting insulin aspart or post-meal fast-acting insulin aspart vs. insulin aspart was 0.01% (–0.16;0.19) and 0.10% (–0.07;0.27), respectively. Following a standardized meal test, ETD for change from baseline in postprandial glucose (PPG) increment at 1 hour was –16.91 mg/dL (–32.15;–1.68) for mealtime fast-acting insulin aspart and 40.16 mg/dL (25.46;54.87) for post-meal fast-acting insulin aspart, both versus insulin aspart. Mean self-measured blood glucose 1-hour PPG increments also showed a trend towards improved PPG control with mealtime fast-acting insulin aspart versus insulin aspart. Rates of overall hypoglycemia (35.56, 37.72 and 38.75 per patient-year of exposure with mealtime fast-acting insulin aspart, post-meal fast-acting insulin aspart and insulin aspart, respectively) and meal-related hypoglycemia were similar between treatment arms. Consistent with findings of onset 8, this analysis confirmed mealtime and post-meal fast-acting insulin aspart provided effective HbA1c and PPG control versus insulin aspart, with similar safety profiles, in Japanese adults with type 1 diabetes.