Introduction: Brucellosis is a common global zoonotic disease with a wide range of complex and nonspecific clinical manifestations that may lead to misdiagnosis and delayed treatment. Osteoarticular involvement is the most common complaint in brucellosis., Objective: This present study aims to describe the clinical and laboratory characteristics and treatment of brucellosis patients with arthritis and sacroiliitis., Methods: This retrospective descriptive study was performed on patients presenting to a teaching hospital in Kermanshah, Iran with a diagnosis of brucellosis from 2011 to 2019. The demographic and clinical characteristics, complications, laboratory findings, and treatment were recorded during the study period. Then, the difference in the collected data was investigated between brucellosis patients with and without arthritis or sacroiliitis., Results: Of 425 patients studied, 130 (30.58%) had osteoarticular involvement. Among them, 41 (9.64%) and 58 (13.6%) patients were diagnosed with arthritis and sacroiliitis, respectively. There were no significant demographic differences between patients with and without brucellar arthritis or sacroiliitis ( p > 0.05). The patients with Brucella arthritis had a significantly higher frequency of arthralgia and radiculopathy ( p ⩽ 0.05). Sacroiliitis was significantly more common in patients with arthralgia, neck pain, and low back pain, positive flexion-abduction-external rotation (FABER) test, radiculopathy, and vertebral tenderness compared to patients without sacroiliitis ( p ⩽ 0.05), while fever and headache were significantly more common in patients without sacroiliitis ( p ⩽ 0.05). The median Wright and 2-Mercapto Ethanol titers were higher in brucellosis patients with arthritis or sacroiliitis versus patients without arthritis or sacroiliitis, but the difference was not significant ( p > 0.05). Synovial fluid had been analyzed in 20 cases. The mean white blood cell count, glucose, and protein level were 3461 ± 2.70 cells/mm 3 , 58.54 ± 31.43 mg/dL, and 8.6 ± 11.85 g/dL, respectively. In 80% of the subjects, neutrophil cells were predominant. There were no significant laboratory differences between patients with and without brucellar arthritis or sacroiliitis, except for a higher median platelet count in patients with arthritis and higher median levels of aspartate aminotransferase (AST) and alkaline phosphatase (ALP) in patients with sacroiliitis. Most cases of arthritis and sacroiliitis were diagnosed with ultrasound (31.8%) and FABER test (79.3%), respectively., Conclusion: Arthritis and sacroiliitis were the two most important and common manifestations of brucellar osteoarthritis with a frequency of 9.64% and 13.6%, respectively. Any complaints of low back pain and radiculopathy as well as the presence of spondylitis in patients should raise suspicion of sacroiliitis. High levels of AST and ALP and a high platelet count may be associated with brucellar sacroiliitis and arthritis, respectively. The use of imaging methods such as MRI and bone scan seems necessary for the diagnosis of sacroiliitis., (© The Author(s), 2024.)