Objective: The aim of this study was to determine the efficacy of mirtazapine, a tetracyclic antidepressant, as monotherapy for the treatment of posttraumatic stress disorder (PTSD). Methods: This multisite, randomized, double-blind, placebo-controlled trial was conducted between April 2006 and November 2010 at the Tuscaloosa and Birmingham Veterans Affairs Medical Centers in Alabama. US military veterans who met DSM-IV criteria for PTSD were randomly assigned to placebo (n = 39) or mirtazapine (n = 39) titrated up to 45 mg/d for an 8-week double-blind period followed by an 8-week open-label phase of mirtazapine treatment. The primary outcome efficacy measure was the Structured Interview for Posttraumatic Stress Disorder (SIP). Secondary measures included other measures of PTSD, depression, and sleep. Analyses of treatment groups involved mixed-model procedures using a random intercept to test the hypotheses that mirtazapine would be more effective than placebo in reducing symptoms of PTSD and depression and improving quality of sleep. Results: Seventy-eight participants were randomized with 61 completing the 8-week controlled phase and 48 completing the open-label phase. No significant differences were observed between groups on the primary outcome of SIP scores during the controlled phase (P = .418). In secondary outcomes, significant improvements per the Clinical Global Impressions-Improvement scale were found for the mirtazapine group compared to the placebo group (P = .041). The 8-week open-label phase demonstrated significant symptom improvement in SIP total score (P = .0003) and in scores on the SIP re-experiencing (P = .0007), avoidance (P = .0309), and hyperarousal (P = .0014) subscales. There were no significant differences in the occurrence of adverse events between groups. Conclusions: This study did not show efficacy of mirtazapine monotherapy in the treatment of PTSD. Identification of more effective treatments, either as monotherapy or adjunctive, for PTSD is imperative.