Importance: Prior literature has explored the prevalence of motor impairments in autistic individuals, but estimates come from clinical samples, convenience samples, or small sample sizes, which limits their generalizability. Better understanding of the frequency of motor milestone delays in autistic individuals could lead to improved early identification and subsequently earlier intervention and better developmental outcomes. Objectives: To determine the prevalence of motor milestone delays in a population-based sample of 8-year-old autistic children and to evaluate if having motor milestone delays is associated with an earlier age of autism evaluation or diagnosis. Design: Cross-sectional study of Autism and Developmental Disabilities Monitoring (ADDM) Network data between surveillance years 2000 and 2016. Data were analyzed from October 2023 to AugustMay 2024. Setting: ADDM Network data are population-based and come from 17 sites across the United States. Participants: Population-based sample of autistic 8-year-old children. Exposures: Binary indicator of motor milestone delays. Main Outcomes and Measures: The prevalence of motor milestone delays among autistic 8-year-old children was calculated. Associations between motor milestone delays and age of autism evaluation or diagnosis were evaluated using linear regression. Covariates included study site, surveillance year, the sum of autism discriminators, intellectual disability, child sex, race, and ethnicity. Results: Among 32,850 children aged 8 years identified with autism by active surveillance, 71.4% met criteria for motor milestone delays. In adjusted regression models, children with motor milestone delays (44.2 months of age; 95% CI, 43.5-44.9) were evaluated for autism significantly earlier (44.2 months of age; 95% CI, 43.5-44.9) than children without motor milestone delays (50.5 months of age; 95% CI, 49.7-51.2). After stratifying by the co-occurrence of intellectual disability (ID), children with motor milestone delays were evaluated for autism earlier than those without motor milestone delays, regardless of having ID. Conclusions and Relevance: This study estimates the prevalence of motor milestone delays among autistic youth and highlights the association between these delays and an earlier autism evaluation, even in children without co-occurring ID. Early identification of autism is a public health priority and examining motor milestone delays may facilitate an earlier autism evaluation, leading to more timely interventions and better developmental outcomes.