Although symptoms of autism are present early in life and early diagnosis can lead to better outcomes, there is a dearth of validated caregiver-report interviews designed for children under the age of 3 years. We developed the Toddler Autism Symptom Inventory, a semi-structured interview designed to assess the presence and absence of skills and symptoms in children aged 12-36 months. Reliability and validity of items and a cutoff score for likelihood of autism spectrum disorder were established. Specificity and sensitivity of this cutoff were confirmed with a cross-validation sample. The Toddler Autism Symptom Inventory effectively identified most children with autism without excessive false positives. The Toddler Autism Symptom Inventory is a developmentally appropriate caregiver interview for use in diagnostic evaluations of children under age 3 years that offers clearly operationalized diagnostic criteria and a cutoff for autism likelihood for very young children.