Efficacy and Safety of KAI-9531 Administered Once Weekly in Participants Living With Obesity or Overweight and Diabetes

The primary objective of this study is to demonstrate that KAI-9531 subcutaneous (SC) injection once weekly is superior to placebo on: * Percent change in body weight * Change in hemoglobin A1c (HbA1c)

Key Inclusion Criteria: * Diagnosis of type 2 diabetes mellitus (T2DM). * Receiving stable therapy for T2DM for 3 months prior to Screening. This includes diet/exercise alone and any oral medication for T2DM treatment except for glucagon-like peptide-1 receptor (GLP-1R) agonist, GLP-1R/glucose-dependent insulinotropic polypeptide receptor (GIPR) dual agonist, or dipeptidyl peptidase-4 (DPP-4) inhibitors. * BMI ≥27 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 type 1 diabetes mellitus (T1DM) or any other type of diabetes except T2DM. * History of diabetic ketoacidosis or hyperosmolar state/coma within 1 year prior to Screening. * History of severe hypoglycemia or hypoglycemia unawareness within 1 year prior to Screening. * 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 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 (GI) motility if taken for \>30 days continually within 3 months prior to Screening. * History of suicide attempt. * History of significant active or unstable Major Depressive Disorder (MDD) or other severe psychiatric disorder within 2 years prior to Screening. * Received treatment with semaglutide, tirzepatide, GLP-1 receptor agonists, GLP-1/glucose-dependent insulinotropic polypeptide (GIP) agonists, glucagon receptor agonists, or other weight loss medications or treatments aside from diet and exercise within 3 months prior to Screening. Note: Additional inclusion/exclusion criteria may apply, per protocol.