There is a growing body of research showing that writers are influenced by motivation-related aspects. This study documents the translation process of the Portuguese version of the Writing Motivation Questionnaire and the construction of two scales to measure self-efficacy for handwriting and story writing. The psychometric properties of these instruments were then explored. Firstly, we examined the factorial structure of these instruments and tested measurement invariance across two independent samples of 202 and 193 third graders. Secondly, we evaluated the reliability of the scales and their convergent/discriminant validity by testing the relationship among them and with external correlates (viz., handwriting fluency and story length and quality). Thirdly, we tested instruments' predictive validity by regressing story length and quality on motives to write, and on self-efficacy for handwriting and story writing. Findings confirmed the multidimensional nature of motivations to write and supported the validity and reliability of the scales developed. Regression analyses showed that competition- and grades-related had, respectively, a negative and positive contribution on story quality. Moreover, self-efficacy for story writing and handwriting contributed, respectively, to story length and quality, above and beyond handwriting fluency. This study provided evidence on the validity and reliability of the instruments under analysis, showing their usefulness to assess motivational dimensions in students as young as 8 years old. Findings join to current research emphasizing the importance of having valid and reliable tools to explore the role of motivation in young children's writing.