We examined relationships between prenatal dietary patterns and child autism-related outcomes, including parent-reported clinician diagnoses of autism spectrum disorder (ASD) and Social Responsiveness Scale (SRS-2) scores, in up to 6084 participants (with analytic samples ranging from 1671 to 4128 participants) from 14 cohorts in the Environmental Influences on Child Health Outcomes (ECHO) consortium. Associations between quartiles of the Healthy Eating Index (HEI-2015), the Alternative Healthy Eating Index modified for Pregnancy (AHEI-P), and the Empirical Dietary Inflammatory Pattern (EDIP), calculated based on reported prenatal diet, and outcomes were examined using crude and multivariable regression (quantile for SRS scores and logistic for diagnosis). In adjusted models, the higher quartile of prenatal HEI score was associated with lower SRS scores (Q4 vs. Q1 β for median quantile = -3.41 95% CI = -5.15, -1.26). A similar association was observed for the AHEI-P score when adjusting for total calories (Q4 vs. Q1 β = -2.52 95% -4.59, -0.45). There were no significant associations of prenatal diet with ASD diagnosis. Findings from this large U.S.-based study do not suggest strong associations between prenatal dietary patterns and ASD-related outcomes, although subtle associations with broader traits suggest the need to further consider how prenatal diet may relate to ASD-related phenotypes.