Discipline-specific writing standards at the secondary level and writing intensive course requirements at the post-secondary level require science teacher and science instructors to teach science writing skills. However, many do not feel equipped in this area, often from lack of professional development or poor perceptions of themselves as writers. Thus, this study investigated science teacher and science instructor science writing instruction efficacy beliefs and identified antecedents to high efficacy. During the first phase, quantitative data were collected from 46 secondary science teachers and 72 post-secondary college instructors using an online survey that included the Teacher Sense of Efficacy Scale and the Writer Self-Perception Survey. The results of the quantitative phase guided the development of the second, qualitative phase, which included analysis of responses to two sets of two open-ended statements and interviews with eight educators: four secondary science teachers and four post-secondary instructors identified as having high science writing efficacy beliefs. Secondary science teachers had a mid- to high-range of efficacy beliefs (M = 6.9, SD = 0.85), whereas post-secondary science instructors had low- to high-range efficacy beliefs (M = 6.3, SD = 1.3). Within both groups, the educators with the highest efficacy beliefs valued science writing, used writing to learn strategies, had experience teaching and integrating writing into their science classes, and faced barriers. Unique to secondary science teachers were having an inner locus of control, being self-directed learners and collaborating with colleagues. Post-secondary science instructors also implemented writing in the discipline strategies and received positive feedback from students regarding writing.