Backgrounds: Environmental factors such as bacterial infections, as well as genetic factors—in particular the human leukocyte antigen (HLA) alleles—have been implicated in the etiology of multiple sclerosis (MS). This study aims to explore the relationship between Helicobacter pylori infection, HLA alleles, and disease severity in Iranian MS patients. Methods: The study population comprised 125 MS patients and 153 ethnically matched healthy controls. Stool antigen test was used to detect H. pylori, and the expanded disability status scale (EDSS) scores were assessed in the patients. HLA-DRB1 and DQB1 alleles and haplotypes were determined in both the patients and the controls. The relationships between H pylori infection, HLA alleles, and EDSS were also analyzed. Results: HLA-DRB1*15 and DQB1*06 alleles families and the DRB1*15~DQB1*06 haplotype were significantly more frequent in MS patients, whereas HLA-DRB1*14 and DRB1*14~DQB1*05 haplotype were less frequent. Of the 125 MS patients, 38 were diagnosed with active H. pylori infection. We found lower frequencies of HLA-DRB1*15 (P = .08) and DRB1*16 (P = .05) alleles and a higher frequency of DQB1*02 (P = .06) in the H. pylori-positive patients. HLA-DRB1*07 was more prevalent in patients with EDSS≤3.0 (P = .06). More severe MS cases (EDSS>3.0) were linked to H pylori positivity (P = .02), disease chronicity (P = .001), receiving non-steroidal anti-inflammatory drugs (P = .02), and female gender (P = .05). Conclusion: These preliminary findings suggest a link between H. pylori infection and the severity of MS H. pylori-positive patients regardless of the type of HLA carriage.