© 2016, MedWorks Media LLC. All rights reserved. Objective: Selective serotonin reuptake inhibitors (SSRIs) remain the firstline treatment for posttraumatic stress disorder (PTSD). However, adjunctive atypical antipsychotics are often used to target residual or refractory symptoms. Asenapine is a novel atypical antipsychotic that possesses a high serotonin (5-HT2A) to dopamine (D2) affinity ratio and alpha-adrenergic antagonism, which may be advantageous in treating PTSD. This pilot study aimed to identify the therapeutic potential of asenapine as an adjunctive treatment for PTSD. Method: Eighteen subjects initiated treatment in this single-site prospective, open-label, 12-week trial of flexibly-dosed asenapine in Veterans with PTSD who had not responded to an adequate course of treatment with an SSRI, venlafaxine, or mirtazapine. Subjects remained on their antidepressant medication and were started on adjunctive asenapine 5 mg sublingual at bedtime, which was gradually titrated to a maximum of 10 mg twice per day, as tolerated. The primary outcome measure was the Clinician Administered PTSD Scale (CAPS) for DSM-IV. Results: Fifteen subjects finished at least 4 weeks and eleven completed the 12 week study. There was a significant and clinically meaningful decrease in CAPS from baseline (77.56 ± 14.48) to week 4 (48.7 ± 30.6), and to week 12 (35.3 ± 19.7). Six participants experienced adverse events possibly related to asenapine; however, only three participants discontinued early due to related adverse events. Conclusion: This pilot study demonstrated that adjunctive treatment with asenapine may provide additional benefit to some patients experiencing residual PTSD symptoms in spite of optimal antidepressant therapy. A larger efficacy study may be warranted.