The Structured Inventory of Malingered Symptomology (SIMS) is a measure of symptom validity that is widely utilized in a variety of settings and populations. Both legitimate and valid symptoms may artificially conflate SIMS scores. The purpose of this analysis was to evaluate the SIMS potential to reflect psychological symptoms of posttraumatic stress disorder (PTSD) and major depressive disorder (MDD) as opposed to negative response bias in veterans. Participants were 483 veterans who completed diagnostic clinical interviews and the SIMS. Descriptive statistics evaluated item-level frequencies. Nonparametric analyses assessed differences between those with and without current diagnoses of MDD, PTSD, a mixed mental health group, and healthy controls. Veterans with PTSD and MDD scored significantly higher than those without these diagnoses on the SIMS Total score and SIMS Affective Functioning (AF) subscale score (p < .001). Thirty-six percent (n = 27) of items were frequently endorsed (≥25%) in the valid MDD subsample, and 40% (n = 30) in the PTSD subsample. Regression analyses showed that MDD was significantly associated with the SIMS Total score (B = 4.34, p < .001) and the SIMS AF subscale score (B = 1.64, p < .001). PTSD was also significantly associated with the SIIMS Total score (B = 6.11, p < .001) and SIMS AF (B = 5.83, p < .001) subscale score. Results suggest that the SIMS may not be an appropriate validity measure for veterans with MDD and/or PTSD.