This study provided validity evidence of the Questionnaire of English Writing Self-Efficacy (QEWSE) based on the five sources of validity evidence described in the Standards for Educational and Psychological Testing (AERA, APA, & NCME, 2014). Items of this measure demonstrated close alignment with the linguistic domain and the writing self-efficacy and self-regulation theories, providing validity evidence based on test content. Confirmatory factor analysis revealed a satisfactory global fit of the four-factor (ideation, skills, use, and self-regulation) model to the data, providing internal structure-based evidence. Significant correlations between self-efficacy ratings, writing proficiency, and writing self-regulated learning strategies supported the validity evidence based on relations to other variables. The reliability evidence was evaluated with consistencies of participants’ responses to items and further supported using Item Response Theory (IRT). IRT analyses suggested that measurement precision was greatest at the lower range of the latent trait scale, which provided justification for the primary use of the instrument to identify students with low writing self-efficacy.