Previous literature has shown that complementary and alternative medicine (CAM) is steadily increasing in autism spectrum disorder (ASD). However, little data is currently available regarding its use, safety, and efficacy in children with ASD. Thus, the purpose of this study is to describe the use of supplement-based CAM therapies in children between the ages of 4 to 17 years with ASD. This population-based, cross-sectional study evaluated children with ASD regarding supplement use. A total of 210 participants were recruited from a variety of sources including educational and physical activity programs, and social media to complete a questionnaire. Primary caregivers provided information on current supplement based CAM use. Data evaluated the proportion of children that used supplement therapies, the types of supplements used, reasons for use, perceived safety, and demographic factors associated with use (e.g., income, parental education, severity of disorder). Seventy-five percent of children with ASD consumed supplements with multivitamins (77.8%), vitamin D (44.9%), omega 3 (42.5%), probiotics (36.5%), and magnesium (28.1%) as the most prevalent. Several supplements, such as adrenal cortex extract, where product safety has not yet been demonstrated, were also reported. A gluten free diet was the most common specialty diet followed amongst those with restrictions (14.8%). Health care professionals were the most frequent information source regarding supplements; however, 33% of parents reported not disclosing all their child’s supplements to their physician. In conclusion, the use of supplement therapies in children with ASD is endemic and highlights the need for further research concerning public health education surrounding safety and efficacy.