Introduction: Rheumatoid arthritis (RA) is a chronic and complex inflammatory disorder characterized by inflammation of the synovial membrane (synovitis), leading to cartilage and bone damage of articular joints. Given the adverse effects and different and non-affective responses of patients to synthetic disease-modifying anti-rheumatic drugs (DMARDs) and nonsteroidal anti-inflammatory drugs (NSAIDs), it is of interest to use medicinal plants exhibiting encouraging therapeutic results and fewer side effects. Objective: This study aimed to investigate the anti-arthritic effect of Inula viscosa (I. viscosa) on the progression of arthritis in an NMRI mouse model. Methods: Mice were divided into six groups (n=6) as follows: normal control, disease control, Diclofenac group (10 mg/kg, p.o. daily), and three groups who were treated daily with 50, 100, and 200 mg/kg IVME (p.o.). Formaldehyde models were obtained by means of sub-plantar administration of 20 µL of formaldehyde (3.75% v/v) into the right hind paws of NMRI albino mice on the first and third of the ten experimental days. Joint diameters were measured; arthritis severity was evaluated by means of inhibition of the paw oedema; histological changes, synovial hyperplasia, and immune cell infiltration were evaluated using histological and immunohistochemical analyses of CD3+, CD20+, and CD68+; and staining markers of ankle joint tissue sections were analyzed with the QuPath v0.5.1 software tool using adaptive thresholding to quantify the percentage staining of positive cells. Results: The administration of I. visocsa (at a low dose of 50 mg/kg) significantly (***p <0.001) ameliorated the induced arthritis severity, resulting in the restoration of the paw diameter, (*p <0.05), reduced hyperplasia of the synovial membrane, and bone erosion, and significantly (*p <0,1) decreased CD68+-staining immune cell infiltration. Conclusion: These findings suggest that I. viscosa leaves have an anti-arthritic property, which is due to the anti-inflammatory effect, probably through cytokine regulation.