Emotion regulation (ER) may be an important transdiagnostic factor for understanding mental and behavioral health given its association with several psychiatric disorders, including autism spectrum disorder (ASD). However, there is limited research on ER in ASD, particularly using biomarkers such as respiratory sinus arrhythmia (RSA). The aim of the current study was to examine RSA among school-aged children with ASD in relation to symptoms of anxiety, executive functioning, and adaptive socialization skills. Results showed decreased RSA in children with ASD (relative to typically developing controls), reflecting decreased parasympathetic nervous system activity. In addition, decreased RSA was associated with increased symptoms of anxiety and lower socialization skills. These findings emphasize the need for interventions targeting emotional and arousal regulation in ASD.