The error-related negativity (ERN) is an event-related potential (ERP) that indexes error monitoring. Research suggests that the ERN is increased in internalizing disorders, such as depression and anxiety. Although studies indicate that the ERN is insensitive to state-related fluctuations in anxiety, few studies have carefully examined the effect of state-related changes in sadness on the ERN. In the current study, we sought to determine whether the ERN would be altered by a sad mood induction using a between-subjects design. Additionally, we explored if this relationship would be moderated by individual differences in neuroticism—a personality trait related to both anxiety and depression. Forty-seven undergraduate participants were randomly assigned to either a sad or neutral mood induction prior to performing an arrow version of the flanker task. Participants reported greater sadness following the sad than neutral mood induction; there were no significant group differences on behavioral or ERP measures. Across the entire sample, however, participants with a larger increase in sad mood from baseline to post-induction had a larger (i.e. more negative) ERN. Furthermore, this effect was larger among individuals reporting higher neuroticism. These data indicate that neuroticism moderates the relationship between the ERN and changes in sad mood.