Synchronous multiple primary cancers of the stomach and kidney are very rare, only 45 cases of synchronous multiple primary cancers of the stomach and kidney had been reported in the literature up until 2020. Thus far, no particular risk factors have been identified. We present a case of synchronous multiple primary cancers of the stomach and kidney in a 67-year-old female presenting with a 3-month history of vomiting and abdominal pain. The diagnosis of gastric adenocarcinoma with signet ring cells was confirmed through upper endoscopy with biopsies, while CT-guided biopsies of the renal tumor confirmed the diagnosis of primary kidney neoplasm.