The present article outlines the development of the parent-reported ADHD symptom infrequency scale (PRASIS), a novel stand-alone questionnaire designed to discriminate between parents exaggerating ratings of ADHD symptoms of their child from parents more accurately reporting symptoms. The PRASIS includes an Infrequency scale (INF) to measure infrequently reported symptoms of ADHD and a clinical scale to measure Diagnostic and Statistical Manual of Mental Disorders-5th Edition (DSM-5) defined ADHD symptoms (ADHD Total). An initial list of infrequency and clinical items was revised over the course of three studies, each conducted on a different sample of participants (n 1 = 154, n 2 = 203, n 3 = 167) recruited via CloudResearch and consisting of mothers of children 4–12 years old. Analyses on the final version of the measure demonstrated good to excellent internal consistency (INF α =.87, ADHD Total α =.94) and high convergent validity of the PRASIS ADHD Total scores with ADHD Rating Scale-5 Total scores (r =.87, p <.001). Omnibus ANOVA comparisons demonstrated excellent group discrimination of both the PRASIS Infrequency scale and the PRASIS ADHD scale (Cohen’s f = 0.81-0-90). Specificity was above the minimum requirement set a priori (≥.80) and resulting sensitivity was similar or higher than other non-ADHD measures in the symptom validity literature. Specificity and sensitivity are reported for multiple cutoff scores, and positive predictive values (PPV) and negative predictive values (NPV) are presented for several base rates.