Chronotype, a person's circadian preference, reflects individuals' natural pattern of alertness levels throughout the day. It has been shown that chronotype contributes to within-day symptom fluctuations in obsessive-compulsive disorder (OCD). However, the underlying mechanism of this effect is not yet clear. Inhibitory control, an executive function crucial for suppressing unwanted behaviors and thoughts, is essential for managing OCD symptoms and is highly influenced by alertness levels. Hence, the current study investigated the impact of chronotype and time of day on inhibitory control in OCD patients, particularly in response to individually tailored symptom-provoking images, using a novel version of the stop signal task. Ninety-three treatment-seeking OCD patients completed an individually tailored Symptom-Provocation Stop-Signal Task, every morning and evening for four consecutive days. Chronotype was assessed using the Morningness-Eveningness Questionnaire (MEQ). Stop-signal reaction time (SSRT) was assessed for neutral and individually tailored symptom-provoking images separately. Patients exhibited longer SSRTs (worse inhibition) to symptom-provoking trials compared to neutral trials. Most importantly, chronotype and time of day significantly interacted to affect SSRTs in the symptom-provocation condition: A significant correlation was found between optimal alertness periods and improved inhibitory control in the symptom-provocation condition. Taken together, these results indicate that inhibitory control in OCD varies with chronotype and time of day. These findings suggest that aligning treatment sessions with patients' chronotype may enhance therapeutic outcomes and that heightening alertness-by increasing the chances of successful inhibition-might be a way to mitigate the vicious cycle of OCD.