Ruminococcus gnavus, a Gram-positive anaerobic coccus, is a common constituent of the human gut microbiota but rarely causes any disease in humans. Herein, we report a case ofR. gnavusbacteraemia in an immunocompromised 73-year-old man with sigmoid colon perforation.R. gnavusis usually reported as Gram-positive diplococci or short chains on Gram staining; however, in our patient, a blood isolate showed Gram-positive cocci in long chains, and organisms from an anaerobic subculture showed morphological diversity. This case provides insight into the morphological diversity ofR. gnavus, which might help with the recognition of these bacteria in the preliminary identification stage on Gram staining.