Background and Context: As computer science (CS) education expands and the need for well-prepared CS teachers grows, understanding what motivates teachers to teach CS can help address challenges to recruiting, preparing, and retaining teachers. Objective: The goal of this work was to develop and validate a scale that measures teachers' motivation to begin teaching CS. Method: Items were field tested with CS teachers. Exploratory factor analysis (n=167) resulted in an 18-item scale measuring four factors of teachers' motivation related to external pressures, external benefits, student benefits, and personal enjoyment. Confirmatory factor analysis (n=495) was used to assess construct validity of the final scale. Findings: Results indicate that the scale is reliable and measures four distinct, but related, constructs of teacher motivation on a continuum from external to increasingly internal. Implications: Researchers and practitioners can utilize this scale to understand teachers' motivations to teach CS and explore how these motivations relate to CS teacher recruitment, preparation, and retention.