Background Little is known about the extent to which embodied digital mediation may support social engagement between children with or without autism (ASD) in free play settings. This study draws on Affordance theory and Constructionism to investigate social play behaviours associated with use of a Tangible User Interface (TUI) during free play. Method The study used a detailed observational and descriptive design. Two groups of children with ASD and two groups of typically developing (TD) children were filmed during a 20-minute play session with either a passive toy, or a digital toy with a TUI. Behaviours were coded according to a scheme based on Parten’s Play States. Data were described in terms of duration, frequency and the likelihood of transition to another state, given the current state. Results For TD children, Parallel and Associative were the most frequently observed Play States across both conditions. For those with ASD, Parallel Play and Non-Play-Related Conversation were the most frequent states in the passive condition, while Parallel and Associative Play were the most common in the TUI condition. This group demonstrated a longer duration of co-operative play with the TUI toy compared to TD children. Both groups showed higher frequencies of social play in the TUI condition. Conclusions Social play states can be effectively mediated by TUIs for both TD and ASD groups. For the ASD group, repetitive behaviour with a TUI may not be inhibitory to social engagement. Practitioners may consider making TUI enabled toys available during free play opportunities.