The purpose of this study is to evaluate the efficacy and safety of redasemtide in adult participants with acute ischemic stroke (AIS). This study consists of 2 cohorts, Cohort A and Cohort B. Cohort A (redasemtide or placebo) will enroll participants who are ineligible for systemic thrombolysis and/or mechanical recanalization therapy. Cohort B (redasemtide or placebo) will enroll participants who are eligible to receive or who have received systemic thrombolysis and/or mechanical recanalization therapy as standard of care.

Key Inclusion Criteria:

• Able to initiate study intervention within 25 hours of stroke onset
• Cohort A: Determined by the investigator to not be eligible for systemic thrombolysis and/or mechanical recanalization therapy (that is, mechanical thrombectomy, local fibrinolytic therapy as per local standard of care) for the current stroke.
• Cohort B: Determined by the investigator to be eligible for systemic thrombolysis and/or mechanical recanalization therapy (that is, mechanical thrombectomy, local fibrinolytic therapy as per local standard of care) for the current stroke.
• Cohort A: If a participant is not eligible for systemic thrombolysis and/or mechanical recanalization therapy, screening and baseline NIHSS score of 6 to 22 is required (inclusive) and must be stable, defined as absence of an increase or decrease of ≥4 points within ≥1 hour to ≤3 hours of screening, and NIHSS score should be 6 to 22 at baseline.
• Cohort B: In a participant who is eligible to receive or has received concomitant systemic thrombolysis and/or mechanical recanalization therapy, the NIHSS score must be 6 to 22 at screening prior to systemic thrombolysis and/or mechanical recanalization therapy and the second/baseline NIHSS score must be 6 to 22 and should be done after systemic thrombolysis and/or mechanical recanalization therapy.
• Medically stable at the time of enrollment except for primary disease and complications associated with it, according to the judgment of the investigator. In addition, hospitalization during the Follow-up Period is not anticipated, and the participant appears likely to be able to complete the study. Medically stable is defined as disease not requiring significant change in therapy for 3 months following enrollment.

Key Exclusion Criteria:

• Any disease or neurological disorder that, in the opinion of the investigator, would interfere with the conduct of the study
• A severe decrease in consciousness level (defined as NIHSS item 1a score 3: Not alert, responds only with reflex motor or autonomic effects, or totally unresponsive, flaccid, and areflexic)
• Disability corresponding to a mRS score of ≥2 before the onset of stroke
• A history of stroke (excluding transient ischemic attack), history of or current intracranial hemorrhage, or head trauma that caused neurological effects within 90 days prior to obtaining informed consent
• Participants with an ischemic stroke in cerebellum and/or brain stem as the main infarction site
• Diagnosis of a current transient ischemic attack
• Unable to undergo either computed tomography or magnetic resonance imaging
• Considered by the investigator to be inappropriate to participate due to a history or complication of serious cardiovascular disease within 1 month of screening (for example, history of acute myocardial infarction, current acute myocardial infarction, uncontrollable heart failure, infective endocarditis requiring treatment, or acute aortic dissection, or requiring or likely to require hospitalization for severe arrhythmia during the study)
• Blood glucose level <50 or >400 milligrams/deciliter after glycemic control
• Systolic blood pressure ≥220 millimeters of mercury (mmHg) or diastolic blood pressure ≥120 mmHg after antihypertensive treatment
• Sensitivity to any of the study interventions, or components thereof, or clinically significant drug or other severe allergy that, in the opinion of the investigator, contraindicates participation in the study
• Use of prohibited concomitant medications or therapies listed in the protocol for the treatment of current AIS
• Participants who have previously received redasemtide
• Participants who have received any investigational product within 90 days of screening

Note: Other protocol-defined Inclusion/Exclusion criteria may apply.