Previous research has suggested that fine motor skills play a role in literacy development. However, it is necessary to confirm the causal nature of the relationships by using intervention studies. In addition, different explanatory mechanisms have been put forward to explain the relation between fine motor skills and literacy development. Executive functions and graphomotor skills are two possible mediators of this link, as they are related to literacy development and enhanced by motor interventions. Accordingly, we aimed to test the benefits of digital training of fine motor skills for children’s fine motor skills as well as for their reading and writing skills, graphomotor skills, and executive functions. A controlled intervention was conducted with 205 fourth graders; for six weeks, half of the children played a digital game designed for motor training, and the other half played a control game training mathematics. Fine motor skills, literacy skills, graphomotor skills, and executive functions were assessed before and after the training. The results revealed that motor training improved not only some motor skills but also some literacy skills, graphomotor skills, and EFs. The findings confirm, for the first time, a causal relationship between fine motor skills practice and spelling by using an intervention design. Spelling improvement was nonetheless not mediated by executive functions or graphomotor skills improvement, suggesting that further studies are needed to better understand the underlying mechanisms.