Yersiniosis is a zoonotic bacterial disease caused by Yersinia enterocolitica (Y. enterocolitica) and Yersinia pseudotuberculosis, which can lead to the development of extraintestinal inflammatory complications, including reactive arthritis. In patients with rheumatoid arthritis (RA), Yersinia infection may mimic a disease flare or coexist with an exacerbation, thereby posing a significant diagnostic challenge. This study presents the case of a 61-year-old woman with seronegative RA in long-term clinical remission, who was hospitalized due to an abrupt worsening of joint symptoms, high fever, and gastrointestinal complaints. Laboratory tests revealed extremely elevated levels of inflammatory markers, including C-reactive protein, ferritin, and D-dimers, as well as neutrophilia with concomitant lymphopenia and significantly elevated titers of anti-Y. enterocolitica IgM, IgA, and IgG antibodies. Imaging studies revealed enlarged mesenteric lymph nodes, further supporting the infectious etiology. A comprehensive clinical and laboratory assessment indicated the coexistence of two parallel pathophysiological processes – an autoimmune exacerbation of RA and Yersinia-associated reactive arthritis. Initiation of the antibiotic therapy in combination with anti-inflammatory and immunomodulatory treatment resulted in significant clinical improvement and gradual normalization of laboratory parameters. This case highlights the importance of considering bacterial infections, particularly those caused by Y. enterocolitica, in the differential diagnosis of RA flares, especially when systemic inflammatory activity is disproportionately high relative to articular manifestations. Early diagnosis and initiation of etiological treatment can prevent disease progression and irreversible joint damage.