A Study of Seltorexant as Monotherapy in Adults and Elderly Participants With Major Depressive Disorder

Inclusion criteria: * Meet diagnostic and statistical manual of mental disorders-5th edition (DSM-5) diagnostic criteria for major depressive disorder (MDD), without psychotic features based upon clinical assessment and confirmed by the SCID-CT diagnosed with first depressive episode prior to age 55 * Experienced at least one MDD episode prior to their current episode based on clinician-confirmed MDD diagnosis/MDE and documented in source records * Current episode of MDD must be a minimum of 8 weeks in duration and maximum of 24 months duration * Must meet one of the following criteria regarding current medication status. 1. Can be presenting for a new episode of MDD with no adequate antidepressant treatment started in the current episode; however, must have been treated with an antidepressant medication in a prior episode for a minimum of 6 weeks at a stable dose at or above the minimum therapeutic level. OR 2. Have taken up to two antidepressant treatments started in the current episode that were stopped (withdrawn), or will be withdrawn (washed out) due to inadequate response as defined by the Massachusetts general hospital antidepressant treatment response questionnaire (MGH-ATRQ) * Body Mass Index (BMI) between 18 and 40 kilograms per square meter (kg/m\^2) * Must be medically stable on the basis of the following performed at screening and double-blind (DB) baseline: physical examination (including a brief neurological examination), vital signs (including blood pressure), and 12-lead electrocardiogram (ECG) Exclusion criteria: * Use of ketamine/esketamine in the current depressive episode (up to 2 doses are allowed prior to screening) * Has treatment-resistant depression (TRD) * Has a primary DSM-5 diagnosis of panic disorder, generalized anxiety disorder, social anxiety disorder, or specific phobia which has been the primary focus of psychiatric treatment within the past 2 years * Current active DSM-5 diagnosis of obsessive-compulsive disorder, posttraumatic stress disorder, anorexia nervosa, bulimia nervosa, or fibromyalgia * Has a history or current diagnosis of a psychotic disorder, bipolar disorder, autism spectrum disorder, borderline personality disorder, or somatoform disorders * Has dementia, any dementing disease, intellectual disability, or neurocognitive disorder * Has a current or recent history of homicidal ideation or serious suicidal ideation within the past 3 months or a history of suicidal behavior within the past 6 months * Has a history of moderate-to-severe substance use disorder including alcohol use disorder according to DSM-5 criteria within 6 months * Has any significant sleep disorder, including but not limited to untreated/uncontrolled conditions * Has known allergies, hypersensitivity, intolerance, or any contraindication to seltorexant or its excipients