Peptostreptococcus anaerobius is a genus of anaerobic Gram + cocci with increasing resistance to the most commonly used antimicrobials. The purpose of this report is to present the successful management of bacteremia due to P. anaerobius in an adult horse. A 19-year-old, Westphalian stallion presented with chronic cough, intermittent pyrexia, depression, and partial anorexia. Laboratory examination revealed neutrophilic leukocytosis, thrombocytopenia, increase of alkaline phosphatase, and hepatic enzymes. The stallion was unresponsive to the initial supportive treatment presenting gradual increase in neutrophilic leukocytosis and azotemia. Ultrasonographic examination and serologic testing were performed to limit differential diagnosis. Blood cultures revealed P. anaerobius, and the antimicrobial regimen was modified accordingly. The horse responded positively to the aforementioned protocol, showing marked improvement of clinical signs and normal WBC values, 7 days after initiation of oxytetracycline therapy. To the best of our knowledge, this is the first report of bacteremia due to P. anaerobius in an adult horse and emphasizes the need of injudicious administration of antimicrobial agents, facing the challenge of multidrug-resistant anaerobic Gram + cocci.