Phase 3 Study of Adjunctive Treatment With Seltorexant in Adult and Elderly Participants With Major Depressive Disorder and Insomnia Symptoms

Inclusion Criteria: Participants in part 1 and direct enrollers to part 2: * Meet DSM-5 MDD, without psychotic features based upon clinical assessment and confirmed by the structured clinical interview for DSM-5 Axis I disorders-clinical trials version (SCID-CT) diagnosed with first depressive episode prior to age 60 * Have had an inadequate response to at least 1 but no more than 2 antidepressants, administered at an adequate dose and duration in the current episode of depression. An inadequate response is defined as less than (\<) 50% reduction but with some improvement (that is, improvement greater than \[\>\] 0%) in depressive symptom severity with residual symptoms other than insomnia present, and overall good tolerability, as assessed by the MGH-ATRQ, and this must include the participant's current antidepressant treatment * Is receiving and tolerating well any one of the following SSRI or SNRI for depressive symptoms at screening, in any formulation and available in the participating country: citalopram, duloxetine, escitalopram, fluvoxamine, fluoxetine, milnacipran, levomilnacipran, paroxetine, sertraline, venlafaxine, desvenlafaxine, vilazodone, or vortioxetine at a stable dose (at therapeutic dose level) for at least 6 weeks * Having a major depressive episode of at least moderate severity, as assessed with 17-item Hamilton Depression Rating Scale, implemented through the Structured Interview Guide (SIGH-D) in a blinded manner at screening and must not demonstrate…