<p>Worry is associated with increased error monitoring in female populations, indexed by a larger error-related negativity (ERN). Although estradiol and progesterone influence anxiety and neural regions involved in error monitoring, little research has examined their role in the association between worry and the ERN. The current study examined how between- and within-person differences in gonadal hormones across the menstrual cycle moderate the association between worry and the ERN in a female sample. Participants (<i>N</i> = 132) provided saliva samples and completed the Penn State Worry Questionnaire daily across a full menstrual cycle. Participants attended up to four in-person laboratory visits during which they completed the Flanker task while concurrent electroencephalography (EEG) was recorded. A significant interaction between estradiol and worry emerged (<i>p</i> = .006), such that worry was associated with an enlarged ERN for those with lower estradiol levels. There was also a significant interaction between progesterone and worry in the opposite direction (<i>p</i> &lt; .001). Higher average worry was associated with an enlarged ERN for those with high average progesterone. No significant effects emerged for within-person changes in worry, estradiol and progesterone on the ERN. Our findings highlight the critical need to understand the association between worry and the ERN within a larger neurocognitive framework to elucidate when and for whom worry’s effects are more impairing.</p>