This article presents the results of a series of studies conducted to develop and validate a self-report measure of the Mindfulness in Teaching Scale. Results from study 1 suggested the presence of two distinct factors measuring teacher intrapersonal mindfulness and teacher interpersonal mindfulness. Both constructs demonstrated strong positive loadings, weak cross-loadings, and adequate internal consistency. In study 2, the validity of these two constructs was confirmed though confirmatory factor analysis. In study 3, we examined the 6-month test–retest reliability, concurrent validity, and predictive validity of scales in a separate sample of 392 teachers. Test–retest reliabilities for both scales were medium–large range. Absolute values of concurrent correlations with measures of teacher burnout and instructional efficacy were in the small-medium range for the teacher interpersonal mindfulness scale whereas teacher intrapersonal mindfulness was unrelated to burnout or instructional efficacy measures. Over a 6-month period, interpersonal mindfulness predicted scores on teacher burnout (emotional exhaustion, depersonalization) and instructional efficacy in working with students (social-emotional and behavior management) whereas intrapersonal mindfulness failed to predict burnout our efficacy measures over this same time period. Results suggest that the Mindfulness in Teaching scale is a promising measure of mindfulness in teachers with good psychometric properties, with the interpersonal subscale predictive of teacher burnout and instructional efficacy over time.