Objective Although clinical trials support equivalence of originator products and biosimilars for etanercept and infliximab, real-world studies among biologics-naïve patients with spondyloarthritis (SpA) are lacking. The objectives were to compare treatment retention in biologics-naïve patients with SpA starting either the originator product or a biosimilar of infliximab and etanercept, and to explore the baseline characteristics of these patients. Methods Patients with SpA (ankylosing spondylitis/non-radiographical axial SpA/undifferentiated SpA), starting infliximab or etanercept as their first-ever biological disease-modifying antirheumatic drug during January 2014-June 2017 were identified in five Nordic biologics-rheumatology registers. Baseline characteristics were retrieved from each registry; comorbidity data were identified through linkage to national health registers. Country-specific data were pooled, and data on infliximab and etanercept were analysed separately. Comparisons of treatment retention between originators and biosimilars were assessed through survival probability curves, retention rates (2 years for infliximab/1 year for etanercept) and Hazard Ratios (HR). Results We included 1319 patients starting infliximab (24% originator/76% biosimilar), and 1015 patients starting etanercept (49% originator/51% biosimilar). Baseline characteristics were largely similar for the patients treated with the originators compared with the corresponding biosimilars. Survival probability curves were highly similar for the originator and its biosimilar, as were retention rates: Infliximab 2-year retention originator, 44% (95% CI 38% to 50%)/biosimilar, 46% (95% CI: 42% to 51%); and etanercept 1-year retention originator, 66% (95% CI 61% to 70%)/biosimilar, 73% (95% CI 68% to 78%). HRs were not statistically significant. Conclusion This observational study of biologics-naïve patients with SpA from five Nordic countries showed similar baseline characteristics and very similar retention rates in patients treated with originators versus biosimilars, for both infliximab and etanercept, indicating comparable effectiveness in clinical practice.