Training studies have shown the positive effects of play tutoring on children's social skills. This study investigates whether the theoretically suggested mechanism of change--social pretend play quality--explains the effect of play tutoring on social pretend play competence and social skills. Twenty-seven Swiss playgroups (N = 214 three- to four-year-olds) participated in a randomized intervention study with three conditions: the intervention group (play tutoring), the material group (half-dose), and the control group (treatment as usual). Weekly treatment sessions took place for six consecutive weeks. Pre-tests, post-tests, and a follow-up were made. Playgroup educators reported on children's pretend play competence and social skills using a questionnaire. Children's pretend play quality during treatment sessions was assessed by standardized behavioral observations. Using latent change models with indirect effects, we investigated whether the intervention effects of play tutoring on children's pretend play competence and social skills are mediated by their social pretend play quality shown during the intervention sessions. The results indicate mediating effects of social pretend play quality on children's change in social pretend play competence and self-oriented social skills (sociability and setting limits). The study supports social pretend play as a beneficial ground to promote social development but also indicates a more complex interplay of different change mechanisms.