Objectives This study compared cognitive functions according to the detailed symptoms and severity of posttraumatic stress disorder (PTSD) and major depressive disorder (MDD) and evaluated the relationship between the characteristics of PTSD symptoms and cognitive functions. Methods This study was a retrospective medical record review study, and the Korean Wechsler Intelligence Scale-Fourth Edition (K-WAIS-IV), Impact of Event Scale-Revised-Korean version (IES-R-K), and the Korean Beck Depression Inventory-II (BDI-II) were used for the correlation analysis. The correlation between the IES-R-K, BDI-II, and K-WAIS-IV in the PTSD and MDD groups was compared, and the correlation between the IES-R-K and K-WAIS-IV subtest in the PTSD group was verified. Results The BDI-II score was significantly higher in the MDD group, and the IES-R-K score was significantly higher in the PTSD group. In the MDD group, there was no significant association between the BDI-II and K-WAIS-IV scores, but in the PTSD group, the association was significant. In the MDD group, the higher the hyperarousal, the lower the working memory and full-scale IQ of the K-WAIS-IV. Whereas, in the PTSD group, the higher the emotional numbness and dissociation of the IES-R-K, the lower the perceptual reasoning and working memory. Particularly, in the PTSD group, there was a significant association between emotional numbness and dissociation and the digit span subtest of working memory. Conclusion The severity of the depression of the PTSD group was associated with the K-WAISIV score. When depressive symptoms were controlled in the PTSD group, emotional numbness and dissociation predicted perceptual reasoning and working memory, particularly, the digit span