Background: Early childhood is a crucial period for language development and building social skills. While distinct, these two processes may impact upon each other. Aims: The current study aimed to identify the directional associations between expressive language ability and prosocial behaviour between three and five years of age. Methods: Participants included 14, 004 children and their families enrolled in the UK Millennium Cohort Study. Children’s expressive language and prosocial behaviour were assessed at three and five years of age utilizing standardized assessments and parent reports. Cross-lagged models were used for data analysis. Results: Better expressive language at three years was associated with increased prosocial behaviour by five years. No support for the inverse direction of association was found. Conclusions: Children’s early ability to effectively express themselves with others may help in building better social relationships by entry into formal schooling. Programming efforts that are tailored towards enhancing positive behavioural growth and social skills in the toddler years are likely to be effective when expressive language is also a targeted component of the toddler’s skill development.