An increasing body of research has focused on the functions of the glutamate receptor ionotropic kainate 4 (GRIK4) gene in treatment for depression, memory, as well as neurodegenerative diseases. It is widely recognized that major depressive disorder (MDD) patients often display cognitive dysfunctions, which do not strictly correlate with the severity of depressive symptoms, and in some individuals persist after clinical remission. However, little is known regarding the particular role of GRIK4 in cognitive functions of patients suffering from a MDD. The current study therefore sought to examine the impact of GRIK4 polymorphism on cognitive functions in MDD patients. A total of 217 MDD patients participated in this study. Their depressive severity was determined with the 17-item Hamilton Depression Rating Scale, and cognitive functions were evaluated using the Stroop Neuropsychological Screening Test, tests of visual reproduction and immediate logical memory, and the verbal fluency test (VFT). All patients were genotyped to determine GRIK4 polymorphisms. Results of VFT revealed statistical differences among all single-nucleotide polymorphism (SNP) genotypes. In the Stroop-color-word test, only rs6589847 was discovered to be statistically different. No significant difference was found in the Stroop-color task scores, the visual reproduction test, or the immediate logical memory test. GRIK4 polymorphism exerted a significant effect on long-term memory retrieval and selective attention, but did not affect immediate memory.