Objectives: To compare the effectiveness of the infliximab biosimilar CT-P13 with originator infliximab over 24 months of follow-up in biological-naïve patients with rheumatoid arthritis (RA) and axial spondyloarthritis (axSpA). Methods: Biological-naïve patients from the Rheumatic Diseases Portuguese Register (Reuma.pt), with a clinical diagnosis of RA or axSpA, who were starting either the infliximab biosimilar CT-P13 or the originator infliximab after 2014 (date of market entry of CT-P13 in Portugal), were included. Patients on biosimilar and originator were compared regarding different response outcomes at 3 and 6 months, adjusting for age, sex and baseline C-reactive protein (CRP). The main outcome was the change in DAS28-erytrocyte sedimentation rate (ESR) for RA and the ASDAS-CRP for axSpA. Additionally, the effect of infliximab biosimilar vs originator on different response outcomes over 24 months of follow-up was tested with longitudinal generalized estimating equations (GEE) models. Results: In total, 140 patients were included, 66 (47%) of which with RA. The distribution of patients starting the infliximab biosimilar and the originator was the same between the two diseases (approximately 60% and 40%, respectively). From the 66 patients with RA, 82% were females, mean age was 56 years (SD 11) and mean DAS28-ESR 4.9 (1.3) at baseline. As for the patients with axSpA, 53% were males, mean age was 46 years (13) and mean ASDAS-CRP 3.7 (0.9) at baseline. There were no differences in efficacy between RA patients treated with the infliximab biosimilar and the originator, either at 3 months (∆DAS28-ESR: -0.6 (95% CI -1.3; 0.1) vs -1.2 (-2.0; -0.4)), or at 6 months (∆DAS28-ESR: -0.7 (-1.5; 0.0) vs -1.5 (-2.4; -0.7)). This was also true for patients with axSpA (∆ASDAS-CRP at 3 months: -1.6 (-2.0; -1.1) vs -1.4 (-1.8; -0.9) and at 6 months: -1.5 (-2.0; -1.1) vs -1.1 (-1.5; -0.7)). Results were similar with the longitudinal models over 24 months. Conclusion: There are no differences in effectiveness between the infliximab biosimilar CT-P13 and the infliximab originator in the treatment of biological-naïve patients with active RA and axSpA in clinical practice.