Synchronous gastric neoplasms are exceptionally rare, with reported coexistence of adenocarcinoma and lymphoma in only 0.08% to 0.1% of gastric malignancies (1). Among them, the association of gastric adenocarcinoma with mantle cell lymphoma (MCL) is exceedingly uncommon with very few cases reported worldwide (2,3). Most reported cases have been diagnosed incidentally during endoscopy performed for nonspecific upper gastrointestinal symptoms. Careful endoscopic evaluation and multiple targeted biopsies are therefore crucial to identify distinct lesions that may otherwise be overlooked, highlighting the indispensable role of endoscopy in detecting such complex presentations and, in cases such as this one, enabling curative treatment through endoscopic resection techniques. We present a 68-year-old man with diabetes mellitus and peripheral arterial disease who developed new-onset dyspepsia. Initial endoscopy revealed a 50-mm ulcerated, infiltrative lesion in the antrum. Biopsies showed an atypical lymphoid infiltrate. A second endoscopy demonstrated additional ulcers in the proximal body and antrum. Immunohistochemistry identified small- to large-sized blastoïd lymphoid cells positive for CD20, CD5, and cyclin D1, with Ki-67 around 70%, and negative for CD3, CD10, and CD23, findings consistent with blastoïd-variant MCL with focal gastric involvement. The surrounding mucosa displayed atrophic follicular gastritis and intestinal metaplasia without Helicobacter pylori. Systemic therapy with R-CHOP was initiated, achieving complete metabolic response after three cycles. Follow-up endoscopic surveillance showed mucosal healing without lymphoid infiltration, a 40 × 40 mm antral scar, and a 0-IIc lesion of 12 mm on the subcardial lesser curvature whose biopsies showed high-grade dysplasia. After multidisciplinary discussion, endoscopic submucosal dissection (ESD) with Flush knife BT 2.0 was performed, yielding a 32 × 20 mm specimen. Histopathology revealed a well-differentiated tubular adenocarcinoma (pT1b, Sm1, 400 µm submucosal invasion) measuring 16 mm, with no lymphovascular invasion and negative lateral and deep margins. The resection was deemed curative, with no indication for gastrectomy. The patient completed R-CHOP for lymphoma and remains under surveillance due to persistent atrophic gastritis and multifocal intestinal metaplasia. At 3-month endoscopic follow-up with biopsies, post-ESD scarring was observed, without new lesions. This rare case highlights the pivotal role of endoscopy across the entire spectrum of care, from early detection and targeted biopsy to curative resection and post-treatment monitoring. Coexistence of gastric MCL and gastric adenocarcinoma may reflect shared mucosal alterations such as atrophy and intestinal metaplasia that predispose to dual neoplasia (1). The use of the Flush knife BT 2.0 facilitated safe and efficient en bloc ESD, consistent with previously reported advantages of this device, including improved hemostasis and maneuverability that allow high R0 resection rates with minimal complications (4). Accurate endoscopic assessment, histopathologic correlation, and multidisciplinary care allowed organ preservation and favorable outcome. Ongoing surveillance is essential, as both entities carry potential for recurrence or metachronous lesions.