Several lines of evidence implicate abnormalities in glutamatergic neural transmission in major depressive disorder (MDD) and treatment response. A high percentage of MDD patients do not respond adequately to antidepressants and are classified as having treatment-resistant depression (TRD). In this study we investigated five GRIK4 variants, previously associated with antidepressants response, in an Italian cohort of 247 MDD no-TRD and 380 TRD patients. We found an association between rs11218030 G allele and TRD. Moreover, significant associations between rs11218030 and rs1954787 and the presence of psychotic symptoms were observed. In conclusion, our data support the involvement of GRIK4 in TRD and in the risk of developing psychotic symptoms during depressive episodes.