Ruxolitinib is a potent JAK1/JAK2 inhibitor that has demonstrated durable improvements in splenomegaly, symptoms, and overall survival in controlled clinical trials in patients with myelofibrosis. The single-arm study reported here was initiated to collect further safety and efficacy data in Japanese patients with myelofibrosis and is the largest study of ruxolitinib in this population. The primary objective was to assess safety. Secondary endpoints included changes in spleen size and patient-reported outcomes. The primary analysis occurred when all patients (N = 51) completed 24 weeks or discontinued. Overall, 86.3% of patients completed treatment; 9.8% discontinued due to adverse events (AEs). Consistent with previous studies, the most common AEs were anemia (62.7%) and thrombocytopenia (29.4%). Furthermore, levels of select immunologic biomarkers remained stable, and no deaths occurred. At week 24, 30.0% of evaluable patients experienced ae50% reductions from baseline in palpable spleen length; 26.0% had ae35% reductions in spleen volume. Additionally, ruxolitinib led to clinically significant improvements in symptoms and quality of life. Overall, findings from this study indicate that ruxolitinib is safe and effective in Japanese patients with myelofibrosis, with these benefits extending to patients with intermediate-1-risk myelofibrosis and to those with low platelet counts.