Background: Play is an essential component of children's development. Children with intellectual disability tend to have poor socioemotional abilities and impaired play. This study examined the effects of a medical/therapeutic clowning play intervention on the playfulness of children with intellectual disability. Method: Two medical clowns facilitated a play intervention in a preschool classroom setting with a total of 52 children with intellectual disability. We compared before and after two groups that received the intervention: group 1 met the medical clowns once a week for 6 months (long-intervention group) and group 2 for 3 months (short intervention group). Children's functioning was assessed using the Vineland Adaptive Behaviour Scales. Children's playfulness was scored using the Test of Playfulness observational assessment. A teachers' focus-group was used to gather additional information on the clowns' work. Results: Children's playfulness increased significantly at the end of the intervention, whereas the improvement in the playfulness scores of group 1 was significantly larger than those of group 2 (t[subscript 50] = -4.82, p < 0.001). The teachers' focus group revealed additional benefits of the medical clowns' work. Conclusion: The results shed light on the play and playfulness of children with intellectual disability and the possible contribution of a clowning play intervention to their development.