This study is testing the effect of a new study medicine NNC0194-0499 in type 1 diabetes. The purpose of the study is to compare the effect of NNC0194-0499 on the blood sugar levels of participants with type 1 diabetes when taken in combination with semaglutide or placebo. All participants will receive standard of care insulin treatment. The study will last for about 36 weeks.

Inclusion Criteria:

• Male or female.
• Age 18-64 years (both inclusive) at the time of signing the informed consent.
• Body mass index 22-35 kilogram per square meter (kg/m^2) (both inclusive) for Part A and 27-35(kg/m^2) (both inclusive) for Part B at the day of screening.
• Diagnosed with type 1 diabetes mellitus greater than equal to (≥) 1 year prior to the day of screening.
• Treated with multiple daily insulin injections and stable insulin dose greater than (>) 90 days prior to the day of screening, as judged by the investigator.
• Use of Continuous glucose monitoring (CGM) device > 180 consecutive days prior to the day of screening.
• Glycated haemoglobin (HbA1c) from 7.2 - 9.0% (both inclusive).
• Considered to be generally healthy (except for mild conditions under stable treatment associated with type 1 diabetes mellitus) based on the medical history, physical examination, and the results of vital signs, electrocardiogram and clinical laboratory tests performed during the screening visit, as judged by the investigator.
• Ability and willingness to wear the Novo Nordisk provided CGM device according to the protocol including replacement of CGM sensor at home.
• Ability and willingness to refrain from wearing own CGM/Flash glucose monitor for the duration of the study, as judged by the investigator.

Exclusion Criteria:

• Any condition, except for mild conditions under stable treatment associated with type 1 diabetes mellitus, which in the investigator's opinion might jeopardise participant's safety or compliance with the protocol.
• Treatment with glucagon-like peptide-1(GLP-1) RA within 90 days before screening.
• Use of any medication with unknown or unspecified content within 90 days before screening.
• Presence or history of cardiovascular disease including stable and unstable angina pectoris, myocardial infarction, transient ischaemic attack, stroke, cardiac decompensation, clinically significant arrhythmias or clinically significant conduction disorders.
• Presence or history of any clinically relevant respiratory, metabolic, renal, hepatic, gastrointestinal, endocrinological conditions (except conditions associated with diabetes mellitus and obesity).
• Recurrent severe hypoglycaemia (more than 1 severe hypoglycaemic event within 180 days) or hypoglycaemic unawareness as judged by the investigator or hospitalisation for diabetic ketoacidosis within 180 days prior to the day of screening.
• Any episode of diabetic ketoacidosis within 90 days before screening.

According to the Novo Nordisk disclosure commitment on novonordisk-trials.com.