To assess the efficacy/safety of ivarmacitinib, a selective Janus kinase (JAK) 1 inhibitor, in patients with moderate-to-severe active rheumatoid arthritis (RA) who had an inadequate response to conventional synthetic disease-modifying antirheumatic drugs (csDMARDs). Patients were randomised (1:1:1) to receive either placebo (n=188), ivarmacitinib 4 mg (n=189) or ivarmacitinib 8 mg (n=189) once daily, with background csDMARDs allowed. After 24 weeks, patients on placebo switched to ivarmacitinib 4 mg for an additional 28 weeks, while those on ivarmacitinib continued their initial dosage. The primary endpoint was the proportion of patients achieving a 20% improvement in the American College of Rheumatology response criteria (ACR20) at week 24. At week 24, ACR20 response rates were significantly higher in the ivarmacitinib 4 mg (70.4%) and 8 mg (75.1%) groups compared with the placebo group (40.4%; both p<0.0001). Both ivarmacitinib doses achieved numerically higher Disease Activity Score 28-joint count C reactive protein of <2.6/≤3.2 response rates compared with placebo. Improvements in efficacy and patient-reported outcomes were sustained through 52 weeks and were noted in patients who switched from placebo after week 24. During the placebo-controlled period, treatment-emergent adverse events (TEAEs) occurred in 81.5% and 90.5% of patients in the ivarmacitinib 4 mg and 8 mg groups, versus 79.3% in the placebo group. Infection-related TEAEs were slightly higher in the ivarmacitinib groups. Ivarmacitinib may offer a potential therapeutic option for patients with RA who have an inadequate response to csDMARDs, with a safety profile that was generally manageable over 1 year of treatment and similar to other JAK inhibitors. NCT04333771.