Sensory reactivity is a new DSM-5 criterion for autism spectrum disorder (ASD). The current study aims to validate a clinician-administered sensory observation in ASD, the Sensory Processing Scale Assessment (SPS). The SPS and the Short Sensory Profile (SSP) parent-report were used to measure sensory reactivity in children with ASD (n = 35) and typically developing children (n = 27). Sixty-five percent of children with ASD displayed sensory reactivity symptoms on the SPS and 81.1% on the SSP. SPS scores significantly predicted SSP scores. We next identified the five SPS tasks that best differentiated groups. Our results indicate that a combination of parent-report and at least the five most differentiating observational tasks may be most sensitive in identifying the presence of sensory reactivity issues.