Background: Identification of key determinants of medication adherence may assist with designing interventions to improve this important parameter. The aim of the study was to determine the rate and predictors of self-reported medication adherence in patients with rheumatoid arthritis (RA) over one-year follow-up. Methods: Socio-demographic, disease, therapy and patient-related factors were obtained from a longitudinal observational cohort of RA patients between May 2014 and June 2016. Medication adherence was measured using self-reported Compliance Questionnaire for Rheumatology (CQR) at baseline, 6 and 12 months. Mixed-effects modelling was used to investigate the relationship between adherence and potential predictors. Result: Of 185 patients invited, 110 were included in the study. The median level of adherence was 71%-74% during the study period. Around 27%-30% of patients achieved > 80% adherence, while roughly one-fifth reported a CQR score within the lower quartile (CQR < 63%). After adjustment for potential confounders, increased age (β = 0.19, P = 0.010), higher self-efficacy (β = 0.89, P = 0.039) and higher medication necessity belief (β = 1.12, P < 0.0001) were associated with better self-reported adherence. Conclusion: We found a moderate level of self-reported adherence over time and significant association with age, self-efficacy and medication necessity belief. The modifiable predictors of adherence found in this study can be used as a potential target for adherence-improving interventions.