This study examined the reliability and validity of the 13-item Classroom Observation Scale as used by teachers and non-clinically trained observers to identify children who more likely than their peers to have autism spectrum disorder in less-resourced preschools. A total of 534 children (ages 2;10 to 4;5, Mean = 3;8) from nine Chinese-language preschools serving families from lower-middle to middle socioeconomic backgrounds in Hong Kong were observed in their first preschool year using the Classroom Observation Scale. The 75 screen-positive children and 55 randomly selected typically developing peers were clinically assessed for autism spectrum disorder 1 year later. The Classroom Observation Scale as used by teachers and non-clinically trained researchers helped to identify preschoolers who were later diagnosed with autism spectrum disorder with odds ratios of 3.11 and 8.66, respectively. This study provided further evidence on the versatility and ecological validity of the Classroom Observation Scale for use by preschool teachers and observers with little or no clinical training in the early identification of children with autism spectrum disorder in community settings.