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Abstract Treatment of paroxysmal nocturnal haemoglobinuria (PNH) includes the monoclonal antibody eculizumab. This randomised, double‐blind, multi‐national cross‐over Phase III study in PNH patients aimed to demonstrate the equivalence of the proposed eculizumab biosimilar SB12 and reference eculizumab (Soliris, ECU). PNH patients with lactate dehydrogenase (LDH) ≥1·5× upper limit of normal were randomised into treatment sequences SB12‐ECU or ECU‐SB12. Four weekly infusions of 600 mg eculizumab were followed by fortnightly infusions of 900 mg until week 50 (ECU/SB12 cross‐over at week 26). Primary endpoints were LDH at week 26 and the time‐adjusted area under the effect curve (AUEC) of LDH over weeks 14‒26 and 40‒52. Among 46 patients (92%) who completed the study, the least squares mean (LSM) difference in LDH at week 26 (34·48; 95% confidence interval [CI] −47·66‒116·62 U/l) and geometric LSM ratio of time‐adjusted AUEC of LDH (1·08; 90% CI 0·95‒1·23) were within pre‐defined equivalence margins. Mean numbers of transfused red blood cell units, other secondary endpoints, pharmacokinetics, and pharmacodynamics were comparable. No patients developed anti‐drug antibodies. Treatment‐emergent adverse events were reported in 72% and 68% of patients in the SB12 and ECU treatment groups, respectively. The results demonstrate equivalence of SB12 to ECU and support SB12‐use in PNH patients.