Teachers play a critical role in school mental health. They aid in the identification and referral of students in need of mental health services and are key players in implementing interventions. Nevertheless, teachers often lack the education and training needed to support youths' mental health. Increasing teachers' mental health literacy (MHL) is recognized as a needed area of focus in improving systems' responses to youths' mental health needs, yet no current tools assess teachers' complete MHL. A measure of MHL for in-service teachers based on current conceptualizations of MHL in the context of schools was developed and tested. The 24-item scale was tested in a sample of 407 full-time teachers in the United States and initial psychometric testing supports its use. The analytic strategy included an exploratory factor analysis (EFA), reliability analysis via Cronbach's a, and convergent validity analysis via zero-order correlations between the Teachers' MHL Scale and theoretically linked constructs. The final scale includes four subscales that measure teachers' understanding of how to promote well-being, recognize potential problems, link students in need to appropriate resources, and the link between well-being and school success. The underlying theory supporting the tool's development and suggestions for its use are discussed.