The Inventory of Problems-29 (IOP-29) is a recently introduced free-standing symptom validity test (SVT) with a rapidly growing evidence base. Its classification accuracy compares favorably with that of the widely utilized Structured Inventory of Malingered Symptomology (SIMS), and it provides incremental validity when used in combination with other symptom and performance validity tests. This project was designed to cross-validate the IOP-29 in a Brazilian context. Study 1 focused on specificity and administered the IOP-29 and a PTSD screening checklist to 154 Brazilian firefighters who had been exposed to one or more potentially traumatic stressors. Study 2 implemented a simulation/analogue research design and administered the IOP-29, together with a new IOP-29 add-on memory module, to nonclinical volunteers; 101 asked to respond honestly, 100 instructed to feign PTSD. Taken together, the results of both study 1 (specificity = .96) and study 2 (Cohen’s d = 2.15; AUC = .92) support the validity, effectiveness, and cross-cultural applicability of the IOP-29. Additionally, study 2 provides preliminary evidence for the incremental utility of the newly introduced, IOP-29 add-on memory module. Despite the encouraging findings, we highlight that the determination of feigning or malingering should never be made off a single test alone.