Antidepressant drugs have often been used as an augmentation strategy for those patients who have demonstrated a suboptimal response to clozapine. The present 16-week double-blind, randomized, placebo-controlled trial study aimed to explore the efficacy and tolerability of duloxetine add-on pharmacotherapy on clinical symptomatology and executive cognitive functioning in a sample of patients with treatment-resistant schizophrenia receiving clozapine. After clinical and neurocognitive assessments, the patients were randomly allocated to receive, in a double-blind design, at a dose of 60 mg per day of duloxetine or a placebo. A final sample of 33 patients completed the study. The results obtained indicate that duloxetine added to stable clozapine treatment showed a beneficial effect on the negative and general psychopathological symptomatology in a sample of treatment-resistant schizophrenic patients. With regard to executive cognitive functions, duloxetine augmentation of clozapine had no significant effects. The findings provide evidence that duloxetine augmentation of clozapine treatment is safe and well tolerated and may be of benefit for patients who are partially responsive to clozapine monotherapy.