In a longitudinal population‐based study of 2,905 children, we investigated if infants' neuromotor development was associated with autistic traits in childhood. Overall motor development and muscle tone were examined by trained research assistants with an adapted version of Touwen's Neurodevelopmental Examination between ages 2 and 5 months. Tone was assessed in several positions and items were scored as normal, low, or high tone. Parents rated their children's autistic traits with the Social Responsiveness Scale (SRS) and the Pervasive Developmental Problems (PDP) subscale of the Child Behavior Checklist at 6 years. We defined clinical PDP if scores were >98th percentile of the norm population. Diagnosis of autism spectrum disorder (ASD) was clinically confirmed in 30 children. We observed a modest association between overall neuromotor development in infants and autistic traits. Low muscle tone in infancy predicted autistic traits measured by SRS (adjusted beta = 0.05, 95% CI for B: 0.00–0.02, P = 0.01), and PDP (adjusted beta = 0.08, 95% CI for B: 0.04–0.10, P < 0.001). Similar results emerged for the association of low muscle tone and clinical PDP (adjusted OR = 1.36, 95% CI: 1.08–1.72, P = 0.01) at age 6 years. Results remained unchanged if adjusted for child intelligence. There was no association between high muscle tone and SRS or PDP. Exclusion of children with ASD diagnosis did not change the association. This large study showed a prospective association of infant muscle tone with autistic traits in childhood. Our findings suggest that early detection of low muscle tone might be a gateway to improve early diagnosis of ASD. Autism Res 2017, 10: 757–768. © 2017 International Society for Autism Research, Wiley Periodicals, Inc.