Patients' beliefs about their prescribed medication are an important factor influencing intentional non-adherence. This study describes rheumatoid arthritis (RA) patients' beliefs about their subcutaneous (SC) biological medication through the Beliefs about Medicines Questionnaire (BMQ), and potential associations. As part of the ARCO study (Study on Adherence of Rheumatoid arthritis patients to subCutaneous and Oral drugs), patients completed the BMQ specifically for their SC biological medication, encompassing a necessity and a concerns scale. The medication possession ratio (MPR) was calculated to assess adherence to the SC biological medication. The BMQ was completed by 321 patients. Between 71.0 and 89.7% of patients agreed/strongly agreed with necessity scale statements, and only 7.2% had low necessity scores. Between 20.0 and 49.8% of patients agreed/strongly agreed with four of five concern scale statements, and 72.3% agreed/strongly agreed with the concern statement regarding long-term medication effects. The percentage with high concerns was 58.9%, and was higher in patients not satisfied with, or with less fulfillment of, tolerability expectations. Non-adherence percentages were, respectively, 13.8 and 13.0% (p = 0.919) in those with high or low necessity, and 16.0 and 10.6% (p = 0.171) in those with high or low concerns. Most patients were categorized as 'ambivalent' (58.5%; high necessity/high concerns) or 'accepting' (36.1%; high necessity/low concerns) of their SC biological medication. The BMQ identified patients' concerns with their SC biological medication. Because patients' concerns could influence non-adherence to medication and future outcomes, physicians should address this issue in the clinic by informing patients and setting clear expectations.