Handwriting is a skill that is constantly used in schools and in the workplace – two environments that are targeted in French legislation passed in 2005 on the integration of people with disabilities. The aim of the present study was to determine the predictive factors for handwriting speed and quality in adolescents and adults with Down syndrome (DS), looking at chronological and developmental age, pen grasp and perceptual-motor processing, which are known to contribute to both the speed and quality of handwriting. Results yielded by simple linear and curvilinear analyses revealed that handwriting speed and quality are related to chronological age, developmental age and perceptual-motor processing. Moreover, multiple linear regressions showed that handwriting quality can be predicted by fine motor coordination. We discuss the influence of different factors on handwriting acquisition in individuals with DS, and the implications in terms of intervention programmes.