This study was designed to validate a multidimensional structure of writing self-efficacy in English as a foreign language contexts, conceptualized in self-regulated learning theory and social cognitive theory. The Second Language Writer Self-Efficacy Scale was developed and evaluated through a series of rigorous validation procedures. The researchers collected data from 609 university students in China. Confirmatory factory analyses through structural equation modeling validated the proposed three-dimensional structure of writing self-efficacy, including linguistic self-efficacy, self-regulatory efficacy, and performance self-efficacy. Model comparisons confirmed the hypothesis that writing self-efficacy is a multidimensional construct, in which the three factors are conceptually related. Internal and composite reliability, convergent validity, and discriminant validity were examined, suggesting satisfactory psychometric properties of the scale. The concurrent validity and predictive validity were checked by examining correlations of writing self-efficacy with motivational beliefs and writing performance. Findings revealed that the three dimensions of self-efficacy had small to moderate correlations with writing performance. Significant correlations were also found between writing self-efficacy and motivational beliefs (e.g., task value, intrinsic goal orientation, extrinsic goal orientation). The findings support a social cognitive view of self-efficacy that acknowledges the interplay of behaviors, personal factors, and environmental conditions. Theoretical and pedagogical implications are discussed.