Abstract Objective Diagnosing Attention-Deficit/Hyperactivity Disorder (ADHD) in adulthood is complicated by the nonspecific nature of attention complaints, which overlap with most forms of internalizing psychopathology as well as elevated rates of invalid symptom overreporting. This study assessed the relationship between posttraumatic stress disorder (PTSD) symptoms and overreporting of both ADHD and general psychological symptoms. Method Data from 308 consecutive adults (mean age = 29.0; mean education = 16.09; 57.5% female, 42.5% male; 35% White, 27% Hispanic, 18% Black, 13% Asian, 7% Other) referred for neuropsychological evaluation of possible ADHD were administered the PTSD Checklist for DSM-5 (PCL-5) and embedded symptom validity tests (SVTs) on the Clinical Assessment of Attention Deficit-Adult (CAT-A) and the Minnesota Multiphasic Personality Inventory-2-Restructured Form (MMPI-2-RF). Patients were classified as overreporting/no overreporting ADHD and general psychiatric symptoms based on elevations on the CAT-A and MMPI-2-RF overreporting SVTs, respectively. Results Logistic regressions showed that higher PCL-5 scores significantly predicted overreporting of both ADHD symptoms on CAT-A (X2 = 7.47, p = 0.006; Classification Accuracy = 85.1%; Nagelkerke R2 = 0.04; B.025; p = 0.007) and psychiatric symptoms on MMPI-2-RF (X2 = 58.76, p < 0.001; Classification Accuracy = 71.4%; Nagelkerke R2 = 0.35; B = -1.92; p < 0.001), with significantly greater variance predicted for overreporting of psychiatric symptoms. Conclusions Higher PTSD symptoms predicted overreporting of ADHD symptoms (CAT-A) and, more significantly, broad psychopathology (MMPI-2-RF). Findings reinforce the necessity of SVTs when evaluating ADHD among adults, particularly in the context of elevated PTSD symptoms, and also indicate that neuropsychologists should consider routinely assessing patients’ trauma history/symptomatology during ADHD evaluations to improve diagnostic accuracy.