Background: Gastrointestinal stromal tumors (GISTs), although exceedingly rare, are the most common mesenchymal tumors in the gastrointestinal (GI) tract. GISTs are often asymptomatic; approximately 10% are found incidentally on imaging or endoscopy for other indications, although GI bleeding, intestinal obstruction, and perforation can occur. We present a case of upper GI bleeding from a duodenal GIST. Proton-pump inhibitor (PPI) therapy resulted in complete endoscopic ulcer healing, yet a discrete mass lesion was identified on endoscopic ultrasound (EUS). Case Report: A 70-year-old female presented with upper GI bleeding, and a duodenal ulcer was identified with esophagogastroduodenoscopy (EGD). Computed tomography (CT) scan of the abdomen and pelvis showed duodenal bulb thickening without clear mass. The ulcer was treated with 1:10,000 concentration epinephrine, injected in 4 quadrants around the ulcer base. The patient’s GI bleeding resolved, and she was discharged with a referral for outpatient EUS follow-up. One month later, EUS showed resolution of the ulcer after PPI therapy but also showed a lesion consistent with GIST that was confirmed by cytologic analysis. The patient was started on imatinib therapy and had no further bleeding. Conclusion: Initial EGD and CT findings could have easily been attributed to duodenal peptic ulcer disease for which follow-up endoscopy is not routinely recommended given the low risk of malignancy. However, because of the high index of suspicion on the part of the referring physicians, duodenal GIST was diagnosed. This case extends the spectrum of the presentation, evaluation, and diagnosis of GISTs and stresses the importance of keeping this rare disease on the provider’s differential, even after routine workup shows no findings of tumor.