Conversion paralysis (CP) is featured by a stress-induced tonic immobility. Although the neural correlates of this psychiatric condition remain largely unexplored, previous reports showed CP to be associated with anterior cingulate cortex (ACC) hyperactivity. We examined the ACC action monitoring function by recording event-related potentials (ERPs) when conversion patients (n = 6) with unilateral arm paresis made speeded responses with their affected and healthy arms on a flankers task. During this task, pre-response ACC action monitoring is reflected in the N2 ERP component, which is increased when incongruent stimuli lead to simultaneously activated competing response tendencies. The results showed that the N2 congruency effects were significantly increased for responses with affected hands compared to healthy hands. There were no such results for post-response monitoring. This study is the first to present electrophysiological correlates of action monitoring in CP and suggests ACC to be hyperactive when movements with affected arms are to be initiated.