A 54-year-old woman on peritoneal dialysis (PD) was hospitalized with peritonitis with a high body temperature, abdominal pain and cloudy peritoneal fluid. She progressively fell into septic-like shock within only 6 hours after onset. The causative bacteria were Streptococcus mitis (S. mitis), part of the normal flora of oral cavity, intestine, female genial tract and upper respiratory tract. S. mitis shows pathogenicity for diseases such as endocarditis, brain abscesses and sepsis in children with malignancy or transplantation. However, S. mitis rarely shows severe pathogenic responses in adults. We report herein a case of fulminant peritonitis caused by S. mitis in an adult PD patient.