The ability to carry out a given task is highly dependent on one's perceived ability to perform that task. Even if teachers are appropriately prepared to teach writing, they may not feel confident in their ability to teach the necessary skills to their students. Self-efficacy, widely ignored by the writing research community, is an important factor to consider in the teaching of student writing. This study developed a new instrument to measure a teachers' writing self-efficacy when teaching writing to students. The Teaching Writing Self-Efficacy Scale (TWSES) explains approximately 52% of the variance of the construct and has over 97% score reliability (α = .973) with 4th through 7th grade educators. The TWSES can be used to assess in-service educators' knowledge gaps in teaching writing for teacher-driven, research-based, professional development.