Objective: Autism spectrum disorder (ASD) is a group of neurodevelopmental disorders with underlying pathogenesis and etiological factors not fully understood. We assumed that galectin-3, which is also linked with inflammatory responses, may be central to the ethiopathogenesis of ASD. Method: The current study consisted of 33 psychotropic medication-naive children with ASD and 32 control subjects. The Schedule for Affective Disorders and Schizophrenia for School-Aged Children, Present and Lifetime Version-DSM-5 (K-SADS-PL-DSM-5) was used to screen healthy controls for psychiatric disorders by a psychiatrist after a physical examination by a pediatrician. The clinical severity of the ASD symptoms has been assessed by the Childhood Autism Rating Scale (CARS). Venous blood samples were collected and serum galectin-3 levels were measured. Results: When the ASD and control groups are compared, the mean galectin-3 level is 417.77 (SD = 200.20) in the ASD group and 243.08 (SD = 64.65) in the control group, and there is a statistically significant difference between the groups (p < 0.001). When examining whether there is a correlation between galectin-3 levels and CARS total scores, no statistically significant correlation was found between them (r = 0.015, p = 0.933). Discussion: In this study, we examined whether serum galectin-3 levels have a relation with ASD in childhood or not. Our findings have indicated that the children with ASD have higher serum galectin-3 levels compared to the controls. However, no significant relationship has been found between serum galectin-3 levels and ASD symptom severity.