Given recent concerns around boys' literacy, this study examined multidimensional writing motivation and engagement among boys. We explored internal and external validity of 11 adaptive (e.g. self-efficacy for writing) and maladaptive (e.g. disengagement from writing) factors of writing motivation and engagement. The sample comprised 781 male Australian high school students (aged 11-18 years). We used confirmatory factor analysis and structural equation modelling to conduct our analyses. Results confirmed the internal structure of the factors and revealed that parent education, and to a lesser degree language background, predicted several motivation and engagement factors. In addition, the adaptive motivation and engagement factors were associated positively with several writing (e.g. enjoyment of writing) and literacy outcomes (e.g. literacy achievement), whereas the maladaptive factors tended to be negatively associated. Invariance was found in the associations among the motivation and engagement factors and the outcomes across several participant subgroups (e.g. by gender, language background). There was also measurement invariance between the study's sample and five randomly drawn samples of students from archived data on domain-general academic motivation and engagement. Combined, our analyses shed light on a domain-specific area of motivation and engagement--writing--that could benefit from an integrative multidimensional examination.