The aim of this post-hoc subgroup analysis, which was based on data from the treat-to-target, 26-week, onset 7 trial, was to confirm the efficacy and safety of fast-acting insulin aspart (faster aspart) versus insulin aspart (IAsp), both in combination with basal insulin degludec, in children and adolescents from Japan with type 1 diabetes (T1D). Of the onset 7 trial population (1 to <18 years; N = 777), 66 participants from Japan (65 Asian and one non-Asian) were randomized to mealtime faster aspart (n = 24), post-meal faster aspart (n = 19), or IAsp (n = 23). Data for the subgroup from Japan were analysed descriptively. Change from baseline in hemoglobin A1c 26 weeks after randomization was 0.23%, 0.74%, and 0.39%, for mealtime faster aspart, post-meal faster aspart, and IAsp respectively. Change from baseline in 1-h post-prandial glucose increment (based on 8-point self-measured blood glucose profiles) showed numerical differences in favor of mealtime faster aspart versus IAsp at breakfast (–30.70 vs. –2.88 mg/dL) and over all meals (–18.21 vs. –5.55 mg/dL). There were no clinically relevant numerical differences between treatment arms in the overall rate of severe or blood glucose-confirmed hypoglycemia. At week 26, mean total insulin dose was 1.119 U/kg/day for mealtime faster aspart, 1.049 U/kg/day for post-meal faster aspart, and 1.037 U/kg/day for IAsp. In conclusion, in children and adolescents with T1D from Japan, mealtime and post-meal faster aspart with insulin degludec was efficacious in controlling glycemia without additional safety concerns versus IAsp.