Play and playfulness have a key role in children's development. Not enough is known about the playfulness of children with autism spectrum disorder (ASD) and its relationship to children's behavioural problems, as well as the role of caregiver behaviours during play in this relationship. The present study examines the moderating role of teachers' emotional availability (EA) in the relationship between behaviour problems and playfulness in children with and without ASD. Sixty-three teacher-child dyads, 31 children with ASD and 32 typically developed children, aged 3-6, were videotaped in play interactions, analysed using the Test of Playfulness and Emotional Availability Scales, and teachers completed the Child Behaviour Checklist. A significant negative correlation was found between behaviour problems and playfulness for children with ASD only. Teachers' EA moderated this relationship, such that high EA eliminated the correlation between behaviour problems and playfulness. Teachers' EA has an important role in promoting interactions with children with ASD.