A link between noncredible responding and low scores on the Grooved Pegboard Test (GPB) is well documented in the clinical literature. However, no specific validity cutoffs have emerged in previous research. This study was designed to examine the classification accuracy of various demographically adjusted cutoffs on the GPB against established measures of performance validity. Analyses were based on a mixed clinical sample of 190 patients (52.1% female) medically referred for neuropsychological assessment. Mean age of participants was 44.1 years, with a mean education of 13.9 years. Criterion measures were the Recognition Memory Test and 3 composites based on several embedded validity indicators. A T score ≤29 for either hand or ≤31 on both hands were reliable markers of invalid performance (sensitivity = .29–.63; specificity = .85–.91). Ipsative analyses revealed that these T score-based cutoffs have zero false positive rates. Failing these cutoffs had no consistent relationship with overall cognitive functioning. A moderate relationship between GPB failure and self-reported anxiety and depression emerged on face-valid screening measures. There was also a moderate relationship between GPB failure and Personality Assessment Inventory scales measuring somatic complaints, borderline traits, antisocial features, and substance use. The newly introduced GPB validity cutoffs were effective at separating credible and noncredible performance on neuropsychological testing. The complex relationship between failing the GPB and emotional problems is consistent with the psychogenic interference hypothesis. It may provide insight into the mechanism behind invalid responding and thus warrants further investigation.