Background The powerful role of self‐efficacy on student achievement is clear. Less clear, however, is our understanding of the specific roles of the different factors of writing self‐efficacy on student writing self‐regulation and success. Aims This study expands our knowledge about student writing self‐efficacy – including its dimensions and the relationship between its dimensions and practical writing outcomes with students from different age groups. Sample Participants were 992 elementary and 518 high school students across 65 teachers and 6 schools in the United States. Methods Data were collected via online questionnaires, one assessing student writing self‐efficacy and the others teachers’ reports of student writing self‐regulation and writing/ELA grades. Results Confirmatory factor analysis (CFA) results demonstrated significantly improved model fit for a three‐factor model of writing self‐efficacy compared to a one‐factor model. A measurement invariance analysis of the writing self‐efficacy scale indicated scalar invariance between the elementary and secondary samples. Structural equation modelling (SEM) results indicated that the second self‐efficacy factor, self‐efficacy for writing conventions, was statistically significantly related to writing/ELA grades for elementary students (β = .33, p < .001) as well as for high school students (β = .27, p < .01). Conventions were also the only factor statistically significantly related to teacher‐reported student writing self‐regulation for both elementary students (β = .26, p < .01) and high school students (β = .37, p < .001). Self‐efficacy for ideation and self‐efficacy for self‐regulation were not found to be significantly related to either student writing/ELA grades or teacher‐reported self‐regulation (all ps > .19). Conclusions Results support the multidimensionality of writing self‐efficacy. In addition, findings emphasize a meaningful link between students’ beliefs in their ability to conform to the rules of writing and their writing self‐regulation and success.