This nationwide cohort study, utilizing the National Health Insurance Research Database and the Registry of Catastrophic Illness in Taiwan, investigated the association between multiple sclerosis (MS) and rheumatoid arthritis (RA). The study matched 1456 newly diagnosed MS patients with 10,362 control patients based on age, sex, and initial diagnosis date. The results showed a higher incidence of RA in MS patients (age-adjusted standardized incidence ratio: 1.72; 95% CI = 1.01–2.91). This association was significant after adjusting for sex, age, and smoking history (hazard ratio: 1.78, 95% CI = 1.24–2.56, P = 0.002). The study concluded that a diagnosis of MS increases the likelihood of subsequent RA, independent of sex, age, and smoking history. The findings suggest that early treatment of RA in MS patients could improve outcomes and reduce mortality.This nationwide cohort study, utilizing the National Health Insurance Research Database and the Registry of Catastrophic Illness in Taiwan, investigated the association between multiple sclerosis (MS) and rheumatoid arthritis (RA). The study matched 1456 newly diagnosed MS patients with 10,362 control patients based on age, sex, and initial diagnosis date. The results showed a higher incidence of RA in MS patients (age-adjusted standardized incidence ratio: 1.72; 95% CI = 1.01–2.91). This association was significant after adjusting for sex, age, and smoking history (hazard ratio: 1.78, 95% CI = 1.24–2.56, P = 0.002). The study concluded that a diagnosis of MS increases the likelihood of subsequent RA, independent of sex, age, and smoking history. The findings suggest that early treatment of RA in MS patients could improve outcomes and reduce mortality.