Background: The infliximab biosimilar CT-P13 has widely received regulatory approval in all indications of reference infliximab, including rheumatoid arthritis (RA) and ankylosing spondylitis (AS). Objective: This retrospective analysis investigated drug survival and long-term safety and effectiveness of CT-P13 in patients with RA or AS in the Republic of Korea. Methods: This non-interventional, retrospective, multicenter analysis collected medical record data for adult patients with RA or AS who received CT-P13 treatment at five Korean referral hospitals (2012–2017). Drug survival and long-term safety were primary outcomes. The secondary outcome was long-term effectiveness, assessed by disease activity measures. Results: Overall, 491 patients were treated with CT-P13 (154 patients with RA [135 infliximab-naïve; 19 switched from reference infliximab]; 337 patients with AS [219 infliximab-naïve; 118 switched from reference infliximab]). Drug survival was similar in naïve and switched patients. Treatment-emergent adverse events (TEAEs) occurred in 31.8% and 29.4% of patients with RA and AS, respectively; incidence was similar in naïve and switched groups. Upper respiratory tract infection, influenza-like illness, and urticaria were the most common TEAEs. Overall, nine (1.8%) patients experienced serious adverse events (SAEs) deemed potentially drug-related; SAEs led to permanent CT-P13 discontinuation in five (1.0%) patients, including three with tuberculosis. Disease activity decreased over time. Conclusion: Up to 5 years of CT-P13 treatment was safe and effective in patients with RA and AS, based on drug survival, incidence of TEAEs, and disease activity. Drug survival and safety were similar in naïve patients and switched groups, supporting switching from reference infliximab to CT-P13.