Inclusion Criteria:

* Diagnosed with type 1 diabetes mellitus greater than or equal to (≥) 1 year before screening.
* Treated with multiple daily insulin injections (daily basal insulin analogue and bolus insulin analogue regimen) ≥ 6 months before screening.
* HbA1c from 7.0-10.0 percentage (%) (53.0-85.8 millimoles per mole (mmol/mol)), both inclusive, at screening confirmed by central laboratory analysis.
* Ability and willingness to adhere to the protocol including performance of self-measured plasma glucose (SMPG) profiles, based on the investigator's judgement.

Exclusion Criteria:

* Known or suspected hypersensitivity to study intervention(s) or related products.
* Previous participation in this study. Participation is defined as signed informed consent.
* Female who is pregnant, breast-feeding or intends to become pregnant or is of childbearing potential and not using adequate contraceptive method.
* Exposure to an investigational medicinal product within 90 days or 5 half-lives of the investigational medicinal product (if known), whichever is longer, before screening.
* Any condition, except for conditions associated with type 1 diabetes mellitus, which in the investigator's opinion might jeopardise participant's safety or compliance with the protocol.
* Anticipated initiation or anticipated change in concomitant medications (for more than 15 consecutive days) known to affect weight or glucose metabolism (e.g., treatment with thyroid hormones, or systemic corticosteroids).
* Known hypoglycaemic unawareness as indicated by the Investigator according to Clarke's questionnaire question.
* Recurrent severe hypoglycaemic episodes within the last year as judged by the investigator.

Note: Other protocol defined Inclusion/Exclusion Criteria may apply.

Eligibility last updated 08/12/2025. Questions regarding updates should be directed to the study team contact.