Although there are many tools for assessing young children’s self-regulation according to varied conceptual definitions and purposes, the purpose of this study was to develop, validate, and norm a Self-Regulation Assessment Scale for Early Childhood (SASEC) for directly evaluating observed behaviors of young children in naturalistic play experiences within the normal preschool environment. An exploratory sequential mixed methods research design was used. The 315 participants included 153 parents and 15 educators for the qualitative component and 147 children ages 3–5 years for the quantitative component. The analytical steps of a qualitative grounded theory research design were applied to adult participant interviews and focus group discussions, which culminated in 12 scale items for measuring a child’s ability to initiate, modulate, and cease behaviors, tasks, or activities of varied complexities, social configurations, and limiting conditions. Children’s SASEC scores were assessed via video recordings of play behaviors in naturalistic settings. Based on factor analysis results, the SASEC items constitute a single construct. According to the results of hierarchical linear modeling and multiple linear regression, preschool children’s SASEC scores can be compared to the SASEC mean and standard deviation regardless of various demographic variables. Implications and recommendations for future work include having early childhood educators, child and youth care practitioners, counselors, parents and families, social workers, behavioral sciences researchers, and policy makers use the SASEC to measure young children’s self-regulation while developing or monitoring the efficacy of generalized enhancement programs and individualized treatment plans. (author abstract)