Objectives Bimekizumab (BKZ) is a monoclonal IgG1 antibody that selectively inhibits interleukin (IL)-17F in addition to IL-17A. BKZ treatment has demonstrated superior efficacy versus placebo (PBO) at Week 16 in biologic disease-modifying antirheumatic drug (DMARD)-naïve patients with active psoriatic arthritis (PsA). Here, we report long-term efficacy and safety to Week 52. Methods BE OPTIMAL comprised a 16-week, double-blind, PBO-controlled period, then 36 weeks treatment-blind. Patients were randomised 3:2:1 to subcutaneous BKZ 160 mg every 4 weeks, PBO with switch to BKZ at Week 16, or reference arm (adalimumab (ADA) 40 mg every 2 weeks). Efficacy outcomes included the American College of Rheumatology (ACR) response criteria 20/50/70, Psoriasis Area and Severity Index (PASI) 75/90/100 in patients with baseline psoriasis affecting ≥3% body surface area and minimal disease activity (MDA); non-responder imputation. Results ACR20/50/70, PASI75/90/100 and MDA responses were sustained with BKZ to Week 52, consistent with results observed at Week 16. Patients who switched to BKZ at Week 16 demonstrated improvements in efficacy with similar results to BKZ-randomised patients by Week 52. To Week 52, 555/702 (79.1%) patients had ≥1 treatment-emergent adverse event (TEAE) during BKZ treatment; 113/140 (80.7%) on ADA. On BKZ, 46 (6.6%) patients had serious TEAEs. 54 (7.7%) Candida infections occurred during BKZ treatment and 1 (0.7%) during ADA; all cases were localised and non-serious. One death occurred in a BKZ-treated patient, unrelated to treatment. Conclusions The efficacy of BKZ in bDMARD-naïve patients with PsA was sustained from Week 16 to Week 52. BKZ was well tolerated with no new safety signals observed. Trial registration number NCT03895203 .