Lower gastrointestinal bleeding poses a significant challenge, especially in patients receiving combined antithrombotic therapy. We report a 72-year-old patient who, following major surgery, developed a miocardial infarction and was started on dual antiplatelet therapy plus low molecular weight heparin. Then, he experienced severe rectal bleeding requiring transfusion. Colonoscopy revealed a circumferential rectal ulcer with a large adherent clot; after clot removal, active oozing from a visible vessel was noted. Argon plasma coagulation failed due to deeply located bleeding. Underwater snare-tip coagulation allowed stable vessel contact and precise energy delivery, achieving complete hemostasis. Self-assembling peptide hydrogel was applied to promote mucosal healing and reduce rebleeding risk. This case highlights the need for adaptable endoscopic strategies in heightened bleeding risk patients.