Patients with inflammatory bowel disease (IBD) have a higher incidence of coronary artery disease (CAD) compared with the general population. Left main coronary artery (LMCA) thrombosis constitutes a very rare but catastrophic manifestation of acute coronary syndrome. Case reports describing young patients with IBD and LMCA thrombosis are scarce. Most importantly, patients with a positive family history of thrombotic events and those with significant genetic or acquired risk factors such as the antiphospholipid antibody syndrome, advanced age, immobilization, pregnancy, oral contraceptive use, obesity, diabetes, and cigarette smoking may have a higher risk of thrombosis among those with active IBD. We describe a 28-year-old man who was admitted for coronary angiography (CA) due to ST-segment elevation myocardial infarction. He had a recent exacerbation of ulcerative colitis. The patient was a smoker without a family history of CAD. Proximal total occlusion of the left anterior descending (LAD) artery and left circumflex (LCX) artery with massive thrombus was shown on CA, whereas a normal dominant right coronary artery delivered collaterals to the LAD artery.