Whereas a number of studies have examined relationships among brain activity, social cognitive skills, and autistic traits, fewer studies have evaluated whether structural connections among brain regions relate to these traits and skills. Uncinate fasciculus (UF) and inferior longitudinal fasciculus (ILF) are white matter tracts that may underpin the behavioral expression of these skills because they connect regions within or provide sensory information to brain areas implicated in social cognition, and structural differences in these tracts have been associated with autistic traits. We examined relationships among self-reported autistic traits, mentalizing, and water diffusivity in UF and ILF in a nonclinical sample of 24 young adults (mean age = 21.92 years, SD = 4.72 years; 15 women). We measured autistic traits using the Autism-Spectrum Quotient, and we measured mentalizing using the Dynamic Interactive Shapes Clips task. We used Tract-Based Spatial Statistics and randomize to examine relationships among fractional anisotropy (FA) values in bilateral ILF and UF, age, cognitive abilities, autistic traits, and mentalizing. Autistic traits were positively related to FA values in left ILF. No other relationships between FA values and other variables were significant. Results suggest that left ILF may be involved in the expression of autistic traits in individuals without clinical diagnoses.