Preschool classrooms provide a unique context for supporting the development of children with social-communication challenges. This study evaluates the feasibility and acceptability of an adapted professional development intervention for preschool teachers (Social Emotional Engagement-Knowledge & Skills-Early Childhood). Social Emotional Engagement-Knowledge & Skills-Early Childhood is a low-resource-intensive, transdiagnostic intervention to address the learning needs of children with a broad range of social-communication challenges in authentic preschool classrooms. The intervention consists of four asynchronous online modules and three synchronous coaching sessions. Participants included one teacher and one target child with social-communication challenges from 25 preschool classrooms from private childcare, Head Start, and public Pre-K programs. Results reveal high levels of Social Emotional Engagement-Knowledge & Skills-Early Childhood feasibility, with 9 out of 10 feasibility benchmarks met: (a) procedures for participant recruitment reliably identified a neurodiverse sample of children with teacher-reported social-communication challenges; (b) teachers showed high levels of program engagement and Social Emotional Engagement-Knowledge & Skills-Early Childhood completion (76%); and (c) results revealed a robust pattern of gains in Social Emotional Engagement-Knowledge & Skills-Early Childhood classrooms and associations among key outcome measures (including active engagement, student–teacher relationship, social-communication competencies). This research prepares a subsequent, larger effectiveness-implementation hybrid trial (Type 1) that investigates the effectiveness of Social Emotional Engagement-Knowledge & Skills-Early Childhood for improving child outcomes and explores facilitators and barriers of program implementation and sustainability. (author abstract)