Children's relationships with parents and peers have been examined as predictors and outcomes, respectively, of theory of mind (ToM). Yet, these two lines of inquiry have remained largely distinct. The current study bridges this gap. Mother–child coordinated interaction and attachment security (continuous rating) were assessed at 2.8 years (N = 128 dyads), ToM was assessed at 3.3, 4.8, and 5.4 years, and child–friend interaction was observed at 4.8 and 5.4 years. Controlling for child expressive language ability at 2.8 years, mother–child coordinated interaction predicted more complex child–friend play and less child–friend conflict via more advanced ToM. No indirect effects from attachment security to friendship quality via ToM emerged. Attachment group status (secure vs. insecure), however, moderated ToM‐friendship associations, such that (a) more advanced ToM predicted more socially complex play with friends, and (b) more conflict with friends predicted more advanced ToM, but only for children classified as secure.