It is well established that writing involves cognitive and motivation processes. Still, both in research and educational contexts, motivation receives less attention than cognition. There is now a growing body of research showing that writers are influenced by motivation-related aspects. This study focuses on the reasons why students write. For that, we developed the Portuguese version of the Writing Motivation. We examined its factorial structure and tested measurement invariance across two independent samples composed of 202 and 193 third graders. We also evaluated the reliability of the scale and tested its relationship with external correlates, namely, self-efficacy for writing, handwriting fluency and story writing skills. Findings confirmed the multidimensional nature of motivations to write and supported the validity and reliability of the scale. This study joins to current research emphasizing the importance of considering the role of motivation in writing. Moreover, it provides useful tools to assess motivational dimensions in school-aged writers, which may deepen our knowledge about writing and guide the development of effective interventions to teach this skill.