A repeated measures single subject design was used to examine the effectiveness of a joint play approach embedded in professional practice, in supporting pretend play for autistic children. Seven autistic children, aged 5–8 years, with a placement within a specialist educational provision, and who demonstrated restricted play, participated in weekly sessions using the Playboxes approach over a period of 3 months. Pre- and post-approach pretend play abilities were assessed using the Symbolic Play Test and the Test of Pretend Play. Every child gained increased age-equivalent scores on the Test of Pretend Play, ranging from + 8 to + 30 months. Pretend Play abilities can support developmental outcomes and incorporation of this approach into regular practice could be of value for autistic children.