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Theory of mind (ToM)—or thinking about the mental states of others—is a cornerstone of successful everyday social interaction. However, the brain bases of ToM are most frequently measured via explicit laboratory tasks that pose direct questions about mental states (e.g. “In this story, what does Steve think Julia believes?”). Neuroanatomical measures may provide a way to explore the brain bases of individual differences in more naturalistic everyday mentalizing. In the current study, we examined the relation between cortical thickness and spontaneous ToM using the novel Spontaneous Theory of Mind Protocol (STOMP), which measures participants’ spontaneous descriptions of the beliefs, emotions and goals of characters in naturalistic videos. We administered standard ToM tasks and the STOMP to young adults (aged 18–26 years) and collected structural magnetic resonance imaging data from a subset of these participants. The STOMP produced robust individual variability and was correlated with performance on traditional ToM tasks. Further, unlike the traditional ToM tasks, STOMP performance was related to cortical thickness for a set of brain regions that have been functionally linked to ToM processing. These findings offer novel insight into the brain bases of variability in naturalistic mentalizing performance, with implications for both typical and atypical populations.