This study is a Phase 2, randomized, double-blind, placebo-controlled, multicenter study of defactinib (VS-6063) in subjects with malignant pleural mesothelioma (MPM) who have not progressed (confirmed partial response or stable disease) following ≥ 4 cycles of treatment with pemetrexed/cisplatin or pemetrexed/carboplatin. Prior to entry and randomization to the study, each subject must have tumor Merlin status(high or low) established by immunohistochemistry performed at a central laboratory. Subjects will be randomized in a 1:1 ratio to receive oral VS-6063 400 mg twice per day, or matched placebo. Randomization will be stratified by tumor Merlin status (high versus low). Progression will be assessed both locally and by central review using the Response Evaluation Criteria In Solid Tumors (RECIST) Version 1.1. Subjects will continue to receive treatment until disease progression or other discontinuation criteria are met. Following documentation of nonfatal disease progression, all subjects will be followed for overall survival by telephone contact every 2 months until end of life or the close of the study.