Prior research has demonstrated that the P300 response may be related to neuropsychological functioning in nonclinical samples. However, the nature of this relation is unclear, and its characteristics in schizophrenia are unexplored. We assessed estimated IQ, neuropsychological tests that assess components of executive functioning, and the P300 component of the event-related brain potential elicited by auditory and visual oddball paradigms in individuals with and without schizophrenia. We observed modest relations between P300 indices and neuropsychological tests purported to assess aspects of executive functioning in both diagnostic groups. Furthermore, multiple regression analyses revealed that whereas control participants with higher estimated IQs demonstrated larger P3 amplitudes to attended auditory targets, the opposite relation appeared evident in schizophrenic participants when variance due to Trailmaking A or a continuous performance task was held constant. Additionally, control participants with higher estimated IQs demonstrated shorter P3 latencies to attended visual targets whereas schizophrenic participants did not when variance due to the Tower of London was held constant. These results suggest diagnostic group differences in the association between P300 and IQ and indicate that investigations designed to explore P300-IQ relations should include measures of executive functioning in their models.