Objective: Pimavanserin is a 5-hydroxytryptamine-2A antagonist and inverse receptor agonist. This phase 2 study examined the efficacy and safety of pimavanserin as adjunctive therapy in patients with major depressive disorder (MDD). Methods: This was a multicenter, randomized, double-blind, placebo-controlled study in patients with DSM-5-defined MDD and an inadequate response to a selective serotonin reuptake inhibitor (SSRI) or serotonin-norepinephrine reuptake inhibitor (SNRI). Using a 2-stage sequential parallel-comparison design, patients were initially randomized in a 3:1 ratio to placebo or pimavanserin added to ongoing SSRI or SNRI therapy; at 5 weeks, placebo nonresponders were re-randomized to placebo or pimavanserin for an additional 5 weeks. Key endpoints were change from baseline to the end of each stage in 17-item Hamilton Depression Rating Scale (HDRS-17) total score and Sheehan Disability Scale (SDS) score. Results: Between December 2016 and October 2018, 207 patients were randomized. For the prespecified pooled Sequential Parallel Comparison Design analyses of Stages 1 and 2, the least squares (LS) mean (SE) difference for the HDRS-17 total score was −1.7 (0.85) (P=.039) and for the SDS score was −0.8 (0.29) (P=.004). At week 5 of Stage 1, LS mean (SE) difference for pimavanserin versus placebo was significant for changes on the HDRS-17 (−4.0 [1.09], P=.0003) and SDS (−1.2 [0.40], P=.0036) with effect sizes of 0.626 and 0.498, respectively. Early and sustained separation of pimavanserin from placebo (P<.05) occurred at 1 week. The most common adverse events with pimavanserin were dry mouth, nausea, and headache. Conclusions: Pimavanserin demonstrated robust efficacy in patients with MDD and an inadequate response to an SSRI or SNRI. Tolerability was consistent with previous experience.