Anxiety often co-occurs with autism spectrum disorder, yet there are few valid and reliable instruments for measuring anxiety in youth with autism spectrum disorder. This article describes the modification of the Pediatric Anxiety Rating Scale for youth with autism spectrum disorder and systematic psychometric evaluation in a well-characterized sample of 116 youth (age: 5–17 years) with autism spectrum disorder and a range of anxiety symptoms. The clinician-administered-Pediatric Anxiety Rating Scale for youth with autism spectrum disorder was modified with input from parents of children with autism spectrum disorder and an expert panel. Unlike many other anxiety measures, the Pediatric Anxiety Rating Scale for youth with autism spectrum disorder is more focused on behavioral manifestations of anxiety versus verbal expression. Results provide preliminary support for the psychometric properties of the Pediatric Anxiety Rating Scale for youth with autism spectrum disorder. The internal consistency of the Pediatric Anxiety Rating Scale for youth with autism spectrum disorder was 0.90. The Pediatric Anxiety Rating Scale for youth with autism spectrum disorder was strongly correlated with parent-report anxiety measures (rs = 0.62–0.68), supporting convergent validity. In support of divergent validity, correlations between the Pediatric Anxiety Rating Scale for youth with autism spectrum disorder and parent ratings of autism spectrum disorder symptoms, social withdrawal, stereotypy, hyperactivity, inappropriate speech, and repetitive behaviors were low (rs = 0.13–0.32). The Pediatric Anxiety Rating Scale for youth with autism spectrum disorder was moderately correlated with parent-reported irritability (r = 0.52). The Pediatric Anxiety Rating Scale for youth with autism spectrum disorder showed good test-retest reliability (intraclass correlation coefficient = 0.75–0.82) and inter-rater reliability (ICCs = 0.70–0.92). Overall, results support the use of the Pediatric Anxiety Rating Scale for youth with autism spectrum disorder for assessing anxiety in youth with autism spectrum disorder. Lay Abstract Many youth with autism spectrum disorder have anxiety, but it can be difficult to assess anxiety with existing measures. We modified the Pediatric Anxiety Rating Scale for youth with autism spectrum disorder and tested the new measure in a group of 116 youth (age: 5–17 years) with autism spectrum disorder. The Pediatric Anxiety Rating Scale for youth with autism spectrum disorder is an interview that a clinician usually completes with the child and parent together. We modified the interview questions and scoring instructions based on feedback from parents of children with autism spectrum disorder and from a panel of experts in autism spectrum disorder and anxiety. Unlike many other anxiety measures, the Pediatric Anxiety Rating Scale for youth with autism spectrum disorder relies less on a child’s verbal expression of anxiety and more on signs that a parent can easily observe. Training clinicians to administer and score the Pediatric Anxiety Rating Scale for youth with autism spectrum disorder was uncomplicated, and raters showed excellent agreement on video-recorded interviews. Youth who were not currently in treatment for anxiety had stable Pediatric Anxiety Rating Scale for youth with autism spectrum disorder scores with repeat measurement over a 1-month period. The Pediatric Anxiety Rating Scale for youth with autism spectrum disorder is a useful clinician-rated measure of anxiety in youth with autism spectrum disorder and fills a gap for assessing anxiety in this population.