Somatic symptom disorder is a psychiatric diagnosis characterized by persistent somatic symptoms and preoccupation with health and symptoms. This chapter describes a series of research findings on its autonomic activity. In the resting state, individuals with somatic symptom disorder showed a trend of lower parasympathetic activity compared with healthy individuals, but the effect size was small to medium, similarly to other psychiatric diagnoses. During psychological events, when women with somatic symptom disorder read about chronic pain topics, they exhibited significantly lower parasympathetic activity compared with healthy individuals. In the process of psychological tasks, women with somatic symptom disorder performed noticeably worse in the emotional Stroop tasks than healthy individuals, accompanied by significantly lower parasympathetic activity. Under diaphragmatic breathing, individuals with somatic symptom disorder did not show more distinctive autonomic activity compared with the resting state. Overall, autonomic activity in the resting state is not sufficient to distinguish somatic symptom disorder from other psychiatric disorders. By adopting a reactivity design, specific psychological themes may amplify the observed lower parasympathetic activity in individuals with somatic symptom disorder. Research on autonomic activity in somatic symptom disorder can also provide insights into its brain mechanisms.