Rheumatoid arthritis (RA) mainly affects polyarticular joints and is characterized by inflammation of the synovial membrane leading to joint destruction. We report on an unusual case of RA presenting as an intra-articular mass invading bone of the wrist joint in a patient with chronic monoarthritis. A 43-year-old man presented with left wrist joint pain and swelling lasting several years. A plain radiograph showed a non-specific osteolytic lesion in the distal ulna but a magnetic resonance image demonstrated an intra-articular irregular mass-like lesion with eccentric bone erosion the distal radioulnar joint. Synovial biopsy detected hyperplasia of the synovial lining cell layer and finger-like protrusions of inflamed and edematous fibrovascular stroma containing dense inflammatory infiltrates, mainly plasma cells, B cells, and CD4+ T cells. Rheumatoid factor and anti-citrullinated protein antibody were highly positive. The patient was diagnosed with RA and treated with disease-modifying anti-rheumatic drugs, showing a good response on further follow-up.