Clostridium hathewayi is a newly described Clostridium species isolated from the feces of healthy human individuals, but its clinical significance is not known. We describe a case of human infection associated with C. hathewayi. The bacterium (strain HKU18) was isolated from the blood culture of a 39-year-old patient with acute gangrenous appendicitis complicated by septic shock. The cells were strictly anaerobic, nonmotile rods that stained gram negative. Conventional phenotypic tests and commercial identification systems failed to identify HKU18 to the species level. 16S rRNA gene analysis showed 1.4% nucleotide difference between the sequence of HKU18 and that of C. hathewayi, indicating that HKU18 was a strain of C. hathewayi. The patient responded to appendectomy and antibiotic treatment. 16S rRNA gene sequencing would be useful in further characterizing the clinical disease spectrum of C. hathewayi.