This study will evaluate the efficacy and safety of 10 mg of oral administration balovaptan once a day (QD) compared with matching placebo in adults with PTSD.

Inclusion Criteria:

• Participants who have a current diagnosis of PTSD as per DSM-5 criteria, with a score of >/=33 on the PCL-5 at screening
• The index trauma event must have occurred in adulthood, i.e., when the participant was >/=18 years old
• The index trauma event must have occurred at least 6 months prior to screening and no more than 10 years prior to screening
• At baseline, either taking a stable dose of a single antidepressant (SSRI or SNRI) for management of PTSD and have been on that medication for >/=6 weeks at that stable dosage and demonstrating residual symptoms of PTSD or prior demonstrated lack of tolerability or lack of efficacy and not taking an antidepressant medication at baseline for >/=6 weeks
• Treatment with permitted medications and/or non-pharmacological interventions at a stable dose for 6 weeks prior to screening
• For women of childbearing potential: agreement to remain abstinent or use contraception

Exclusion Criteria:

• Participants who are experiencing ongoing exposure to traumatic events within 3 months of screening
• Participants who are pregnant or breastfeeding, or intending to become pregnant during the study or within 14 days after the final dose of study drug
• Clinically significant psychiatric and/or neurological conditions, which may interfere with the assessment of safety or efficacy endpoints
• Substance use disorders during last 12 months
• Significant risk for suicidal behaviour
• Epilepsy or seizure disorder considered not well controlled within the past 6 months or changes in anticonvulsive therapy within the last 6 months
• Clinical diagnosis of peripheral neuropathy
• Within the last 2 years, unstable or clinically significant cardiovascular disorders
• Positive serology results for hepatitis B surface antigen (HBsAg), hepatitis C virus (HCV) antibody, or human immunodeficiency virus (HIV) 1 or 2
• Moderate or severe hepatic or renal impairment
• History of coagulopathies, bleeding disorders, blood dyscrasias, hematological malignancies, myelosuppression (including iatrogenic)
• Medical history of malignancy, if not considered cured
• Participants who have received treatment with investigational therapy within 8 weeks prior to randomization
• Known hypersensitivity to balovaptan, its components, or any of the excipients used in the formulation