Patients with rheumatoid arthritis (RA) from different ethnic groups present elevated levels of antibodies against Proteus mirabilis. This finding implicates P. mirabilis in the development of RA. The aim of this study was to investigate the importance of P. mirabilis in the etiopathogenesis of RA in Greek RA patients. In this study, 63 patients with RA and 38 healthy controls were included. Class-specific antibodies IgM, IgG, and IgA against three human cross-reactive and non-cross-reactive synthetic peptides from P. mirabilis-hemolysin (HpmB), urease C (UreC), and urease F (UreF)-were performed in all subjects, using the ELISA method. RA patients had elevated levels of IgM, IgG, and IgA antibodies against HpmB and UreC Proteus peptide which are significantly different compared to healthy controls: p = 0.005, p < 0.001, and p = 0.003 and p = 0.007, p = 0.002, and p < 0.001, correspondingly. Also, elevated levels of IgM, IgG, and IgA antibodies against the UreF Proteus peptide-which are non-cross-reactive with human tissue antigens-were observed and their significant difference compared to healthy controls (p = 0.007, p < 0.001, p < 0.001). Anti-peptide antibodies in RA patients showed a significant correlation with rheumatoid factors (Rf), erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP), especially when patients were divided into subgroups according to the receiving treatment. Greek RA patients present elevated levels of antibodies against P. mirabilis antigenic epitopes, such as in North European populations, albeit Greek RA patients presenting the cross-reaction antigen in a low percentage. These results indicate that P. mirabilis through the molecular mimicry mechanism leads to inflammation and damage of the joints in RA.