<p>Assessing for autism spectrum disorder (ASD) is challenging, especially in clinical populations like Attention-Deficit/Hyperactivity Disorder (ADHD), which is known to have significant overlap in symptoms and high rates of comorbidity. The current systematic review examined the utility of diagnostic and screening tools in accurately ruling out ASD among youth with ADHD. The literature was reviewed for studies that examined the psychometric properties of ASD measures among school-aged children with a confirmed diagnosis of ADHD. Data from the included ASD measures were synthesized narratively. Risk of bias and applicability were evaluated with the QUADAS-2, a quality assessment tool for reviews of diagnostic test accuracy. In total, 20 studies evaluating eight screening and diagnostic instruments were located. 67% of the studies reported adequate specificity (0.8 or higher) in ruling out ASD, although there was very limited research (<i>n</i> = 5) on the performance of diagnostic measures. The Social Communication Questionnaire was the most studied measure (<i>n</i> = 8) and showed consistently good psychometric properties across studies. Quality ratings indicated high risk of bias in the sample composition (e.g., 60% of studies underrepresented girls and 75% underrepresented those with intellectual disability) and method of diagnosis (e.g., 60% of studies failed to use best estimate clinical diagnosis). This review highlights the need for more research investigating the performance of ASD measures, especially among girls with ADHD and individuals with co-occurring intellectual disability. Further, research should also emphasize methodologically rigorous practices, such as best estimate clinical diagnosis and use of blinding.</p>