Introduction: Understanding the disease among rheumatoid arthritis (RA) patients is an important aspect of treatment, enabling them to make crucial decisions and undertake daily activities aimed at controlling the disease. Objective: The aim of the research was to examine the knowledge of RA patients and their family members about RA. Methods: A cross-sectional study conducted from January to May 2024 included 126 respondents divided into two groups. The first group consisted of 64 RA patients treated at the Department for Rheumatology, Physical Medicine, and Rehabilitation at UHC Sestre Milosrdnice. The second group comprised 62 family members (first-degree relatives or spouses) living in the same household. Patients' knowledge levels and their physical functioning were measured using the self-assessment questionnaires: Patient Knowledge Questionnaire in Rheumatoid Arthritis (PKQ-RA) and Health Assessment Questionnaire (HAQ). Data were analysed using descriptive and non-parametric statistical methods. Results: Family members scored lower in knowledge about RA compared to patients, and this difference was statistically significant at the 5% risk level (t=2.165; p=0.032). RA patients achieved the lowest average knowledge score on the subscale related to joint protection and energy conservation (2.9 out of 7 points) and the highest on the subscale related to physical exercises (4.87 out of 7 points). Gender, age, education level, and disease duration of RA patients did not correlate with their knowledge levels. There was no significant correlation between functional status and knowledge level (correlation close to zero, r=0.032; p>0.05). Conclusion: Knowledge levels of patients with rheumatoid arthritis vary from poor to good depending on the area. Family members have lower knowledge levels compared to patients. No significant correlation between the functional status and the knowledge level of patients was found.