There are several pieces of evidence regarding the role of bacteria, such as Streptococcus bovis/gallolyticus in the etiology of gastrointestinal diseases such as colorectal cancer (CRC) and inflammatory bowel disease (IBD). Therefore, the aim of this study was to detect S. gallolyticus subsp. gallolyticus (Sgg) in fecal samples of CRC and IBD patients by culture and molecular methods, in Ahvaz, southwest of Iran. A total of 106 fecal samples were collected from 22 CRC patients, 44 IBD patients, and 40 healthy individuals. The prevalence of Sgg was investigated by culture and polymerase chain reaction (PCR) with specific primers for sodA gene. The results of the stool culture showed that the overall prevalence of Sgg was 9 (13.6%) out of 66 patients. Meanwhile, the number of Sgg isolated from IBD and CRC patients was 7 (15.9%) and 2 (9%), respectively. The bacteria were not isolated from any of the control groups. On the basis of PCR, S. gallolyticus was detected in 24 (36.4%) out of 66 patients. Meanwhile, the number of IBD patients with positive sodA gene was 15 (34.1%) out of 44 cases. In CRC patients, the sodA gene was detected in 9 (40.9%) of 22 cases. Two (5%) of the specimens in the control group had the sodA gene. According to our results, S. gallolyticus subsp. gallolyticus might be involved in CRC and IBD pathogenesis. More investigation with different samples in the various areas might be shaded light on these results.