Retroperitoneal abscesses are uncommonly encountered clinical entities and they represent serious surgical infections associated with significant mortality rates because of their insidious clinical manifestations and diagnostic difficulty. The source of retroperitoneal infections is usually an organ contained within or abutting the retroperitoneum, usually the kidney and the microorganisms most commonly isolated are gram-negative bacilli. Gram-positive cocci, mainly staphylococcal species and rarely streptococcal species, are a less common cause of retroperitoneal abscess and are usually isolated in cases of hematogenous spread. Treatment of retroperitoneal abscesses includes identification and treatment of underlying conditions, intravenous antibiotics and adequate surgical drainage of all well-defined collections. We present a rare case of retroperitoneal abscess caused by monomicrobial Streprococcus mutans infection and discuss the possible pathogenesis, clinical presentation, diagnosis and treatment.