One in 5 preschool-aged children (under age 5) have a serious mental health difficulty that can have lasting negative effects. Most children under age 5 (63%) attended preschool, day care centers, Head Start, or another prekindergarten program. This leaves early childhood programs well-positioned to support the wellbeing of our earliest learners if they could offer gold standard multi-tiered systems of support for mental health care. However, there is not currently a measure for preschool teams to self-assess and improve their school mental health quality. To address this gap, we adapted an existing measure of school mental health quality for K-12 schools to preschools. Measure development included initial modifications by a preschool quality workgroup, a modified Delphi procedure including 3 rounds of data collection, and a Principal Component Analysis. The resulting measure, the School Mental Health Quality Assessment – Preschool (SMHQA-PS) consists of 46 Likert-style items covering 6 domains of preschool mental health system. Practice and Policy: This measure provides an evidence-informed tool to assist preschool mental health teams in advancing the quality of mental health supports in preschools while creating consistent language for families, school teams, researchers, and policymakers to use across school mental health programs in Pre-K-12 schools. (author abstract)