Poor adherence to medication in schizophrenia spectrum disorders leads to inadequate symptom control. Adherence therapy (AT) is an intervention that seeks to reduce patients' psychiatric symptoms by enhancing treatment adherence. We aimed to systematically review the trial evidence of the effectiveness of AT on improving clinical outcomes in these patients.Systematic review and meta-analysis of published RCTs. We included studies testing AT as an adjunct intervention against treatment as usual or a comparator intervention in the general adult psychiatric population. The primary outcome of interest was improvement in psychiatric symptoms.We included six studies testing AT in schizophrenia spectrum disorders published since 2006. A meta-analysis showed AT significantly reduced psychiatric symptoms compared to usual treatment over a follow-up period of less than 1 year. We found no significant effects of AT on patients' adherence and adherence attitudes.AT is an effective adjunctive treatment for people with schizophrenia spectrum disorders.CRD42015016779.