We report a case of bloodstream infection due toRuminococcus gnavus(R. gnavus) associated with pelvic abscess in a 74-year-old female patient undergoing radiotherapy for cervical cancer. Gram staining of positive anaerobic blood cultures revealed short chains of gram-positive cocci. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry was performed directly on the blood culture bottle, and 16S rRNA sequencing identified the bacterium asR. gnavus. There was no leakage from the sigmoid colon to rectum on enterography, andR. gnavuswas not found in the culture of her pelvic abscess. After the administration of piperacillin/tazobactam, her condition markedly improved. This patient withR. gnavusinfection demonstrated no gastrointestinal involvement, whereas past published cases reported diverticulitis or intestinal damage. It is possible that bacterial translocation ofR. gnavusoccurred from the gut microbiota, due to damage to the intestinal tract caused by radiation.