This study highlights the potential of play to support young children's mark-making and writing and, in turn, shows how writing can become a driver of play. By detailing elements of this symbiotic relationship, the intention was for teachers as practitioner-researchers to deepen their professional knowledge of how best to support young children in meaningful writing activity, thereby countering policy-led reductive understandings of both play and literacy. During a year-long research project, six English reception class teachers gathered photographs, video and multimodal observations of children and used these to analyse how multimodal mark-making and writing occurred as part of play in their classrooms. From this analysis, the concept of 'playful writing' was developed. This identified three interconnecting characteristics that mark-making and writing has as part of play activity: social function, multimodal movement and material possibilities. The findings of the study indicate that young children's classroom play stimulates the multimodal, social and material dimensions needed for the creation of meaningful and transformative writing. The study highlights the importance of harnessing teachers' expertise in paying close attention to children's playful literacy, in order to create a strong knowledge base from which to make the best decisions about literacy practices with young children.