Although previous studies have shown that patients with multiple sclerosis (MS) may exhibit impaired decision making (DM), the specific neuro-cognitive processes that underlie this deficiency remain unknown. Inefficient DM may occur due to poor option assessment or a deficit in the evaluation of choice outcomes. This study uses, for the first time, the event-related potential (ERP) technique to analyze these DM subprocesses in patients with MS. Sixteen MS patients and nineteen control subjects performed a DM task based on the Iowa Gambling Task while a digital EEG was recorded. The P3b was analyzed as an index of option assessment, and both the feedback-related negativity (FRN) and P3b were considered markers for choice outcome evaluation. We found that MS patients made a larger number of unfavorable choices during the DM task (P = 0.02), with no differences between groups for reaction times. There were no significant differences in P3b for option assessment between the groups. In the control group, the type of feedback (gain, loss, or nothing) modified the FRN (F = 3.72, P = 0.033) and the P3b (F = 3.15, P = 0.059). This effect was not observed in the MS group. Also, FRN latencies were shorter in the MS group compared to controls (P = 0.047). Finally, lesions in temporal regions revealed by MRI were associated with DM both in behavioral and electrophysiological terms. We conclude that the DM impairment of patients with MS may be due to a deficit in their evaluation of choice outcomes or a general alteration in emotional reactivity.