Here, we present a case of an 84-year-old woman who developed obstructive jaundice and was diagnosed with nonoperable adenocarcinoma originating from the ampulla of Vater, a lethal disease with a median overall survival of less than a year. Her tumor was examined by next-generation sequencing, which showed BRAF and NRAS mutations. To target these mutations, a MEK inhibitor was chosen for treatment. The patient has been treated with a MEK inhibitor for the last 12 months since diagnosis, with clinical and laboratory improvement and manageable side effects. PET-computed tomography imaging has shown stable disease or improvement in the primary and metastatic lesions. This is the first case report of an ampulla of a Vater cancer patient with NRAS and BRAF mutations, identified in next-generation sequencing, and treated successfully with a MEK inhibitor.