A Study of Seltorexant in Participants With Probable Alzheimer's Disease

Prior treatment

• Participant has a clinically significant acute illness within 7 days prior to study intervention administration
• Participants with a history of delirium within 30 days prior to or during screening
• Participant with a cause of agitation that is not secondary to dementia (such as pain) or significant history of aggression prior to dementia based on investigator judgment
• Participants who are not stable on concomitant medications or take prohibited medications

Other illness

• Participant fulfils diagnostic criteria for non-Alzheimer's Dementia: example, Frontotemporal Dementia (FTD), Diffuse Lewy Body Dementia (DLBD), and post-stroke dementia, based on clinical history. (Participants may be included with mixed AD/vascular dementia)

• Participant has received a diagnosis of probable Alzheimer Disease (AD) (Diagnostic and Statistical Manual of Mental Disorders-5 \[DSM-5\]) with the following characteristics at screening: Clinical Dementia Rating (CDR) global score greater than or equal to (\>=) 1; Mini-Mental State Examination (MMSE) total score of 10 to 24 (inclusive)
• Participant meets the criteria of a syndrome diagnosis of agitation based on International Psychogeriatric Association (IPA) consensus clinical and research definition of agitation in cognitive disorders for at least 2 weeks before screening
• Participant meets the criteria of Neuropsychiatric Inventory (NPI-12) Agitation/Aggression (A/A) domain score \>= 4 with frequency score \>= 2 at screening and baseline with no more than 35 percent (%) of improvement in NPI-12 A/A domain score from the screening to baseline assessments
• Female participants must be postmenopausal before study entry (amenorrhea for at least 12 months)
• Body Mass Index (BMI) within the range 18-40 kilograms per square meter (kg/m\^2) (inclusive)