The primary objective of this study is to demonstrate that KAI-9531 subcutaneous (SC) injection once weekly is superior to semaglutide SC once weekly and to placebo SC once weekly on percent change in body weight.

Key Inclusion Criteria:

• BMI ≥35 kilograms per square meter (kg/m^2).
• History of at least 1 self-reported unsuccessful effort to lose weight with diet and exercise within the prior 6 months.

Key Exclusion Criteria:

• Current diagnosis or history of diabetes mellitus.
• Started medications within 3 months prior to Screening that may cause significant weight gain, including, but not limited to, tricyclic antidepressants, atypical antipsychotics, and mood stabilizers.
• Unstable weight defined as self-reported change in body weight exceeding 5% within the 3 months prior to Screening.
• Family or personal history of multiple endocrine neoplasia Type 2 or medullary thyroid cancer.
• Uncontrolled hypertension or unstable cardiovascular disease.
• History of chronic or acute pancreatitis.
• Known clinically significant gastric-emptying abnormality or chronic treatment with medications that directly affect gastrointestinal motility.
• History of suicide attempt.
• History of significant active or unstable Major Depressive Disorder (MDD) or other severe psychiatric disorder.
• Received treatment with semaglutide, tirzepatide, glucagon-like peptide-1 receptor (GLP-1R) agonist, GLP-1/glucose-dependent insulinotropic polypeptide (GIP), or glucagon receptor agonist within 3 months prior to Screening.

Note: Additional inclusion/exclusion criteria may apply, per protocol.

The participant and investigator will be blinded for participants randomized to KAI-9531 or placebo until the study is complete. Semaglutide will be provided open-label.