Autism Spectrum Disorder (ASD) is an intricate neurodevelopmental disorder characterized by impaired social functioning and communication, repetitive behavioral patterns, and specialized interests (Olesova et al., 2020; Osredkar et al., 2023). Despite the efforts of modern science, the biological origin of ASD is unknown, and no known biomarker for ASD currently exists (Olesova et al., 2020; Osredkar et al., 2023). Indoxyl sulfate has been identified as a toxin associated with ASD and its related symptomology in addition to a number of other conditions, including chronic kidney disease, acute kidney injury, heart failure, Parkinson’s disease, and various mood disorders (Cao et al., 2015; Cassani et al., 2015; Karbowska et al., 2020; Zhao et al., 2013). This article will review what is currently known about indoxyl sulfate in relation to ASD and its comorbidities in an attempt to determine the validity of indoxyl sulfate as a potential biomarker for ASD. Articles for the purposes of this review were collected via Google Scholar, PubMed, and the ASU Library using key words such as “indoxyl sulfate,” “Autism,” and “indican,” and chosen based on relevancy. Through this review, indoxyl sulfate was identified as a potential physiological biomarker for a subset of ASD, with additional research required to verify the findings presented. The identification of a biomarker for ASD could change the current methods of testing for ASD, greatly improving our understanding and treatment of the disorder.