The purpose of this clinical study is to find out if UBT251 is safe and effective for treating people who are living with overweight or obesity. Participants will get either UBT251 (the treatment being tested) or Placebo (a treatment that has no active medicine in it), which treatment participants get is decided by chance.

Inclusion Criteria:

1. Male or female (sex at birth).

2. For Part C: Japanese, Chinese or non-Asian participants (all self-reported):

   • For Japanese participants: both parents of Japanese descent.
   • For Chinese participants: both parents of Chinese descent.
   • For non-Asian participants: both parents of non-Asian descent (non-Asian is defined as of countries outside of Asia).

3. Age at the time of signing the informed consent:

   1. For Part A: 18-55 years (both inclusive)
   2. For Part B: 18-65 years (both inclusive)
   3. For Part C: 18-55 years (both inclusive).

4. BMI at screening (overweight and obesity should be due to excess adipose tissue, as judged by the investigator):

   1. For Part A: 27.0-39.9 kilogram per meter square (kg/m^2) (both inclusive)
   2. For Part B: 30.0-50.0 kg/m^2 (both inclusive)
   3. For Part C: 24-34.9 kg/m^2 (both inclusive)

5. Considered eligible based on the medical history, physical examination, and the results of vital signs, electrocardiogram (ECG), and clinical laboratory tests performed during the screening visit, as judged by the investigator.

Exclusion Criteria:

1. Known or suspected hypersensitivity to study intervention(s) or related products.
2. Treatment with any marketed product containing compounds with glucagon-like peptide 1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP) or glucagon receptor agonism within 90 days before screening.
3. Any condition, unwillingness or inability, which in the investigator's opinion might jeopardise the participant's safety or compliance with the protocol.

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