Navitoclax (oral B-cell lymphoma-2 family protein inhibitor induces apoptosis of malignant cells in myelofibrosis (MF). We present pooled cohort 1 results from the phase 2 REFINE trial, which evaluated navitoclax plus ruxolitinib (NAV+RUX) for patients with relapsed/refractory MF with suboptimal response to RUX (≥10 mg twice daily stable dose for ≥12 weeks [cohort 1a] or ≥24 weeks [cohort 1b]). Cohort 1a received add-on NAV 50 mg/d, with escalation to ≤300 mg if platelet count was ≥75 × 10/L. Cohort 1b received NAV 100 or 200 mg/d if platelet count was ≤150 or >150 × 10/L, respectively. The primary end point was spleen volume reduction of ≥35% (SVR) at week 24. Secondary end points included ≥50% total symptoms score (TSS) reduction at week 24, bone marrow fibrosis (BMF) grade changes, anemia response, and safety. In total, 125 patients received ≥1 dose of NAV+RUX. With median follow-up of 21 months, SVR rate was 23% at week 24 and 39% at any time on study (median duration: 11 months). TSS rate was 24% at week 24 and 46% at any time on study. BMF improved by ≥1 grade, any time on study, in 39% of patients. Anemia responses were achieved in 23% of patients. Median overall and progression-free survival were 52.3 and 22.1 months, respectively. No new safety signals were observed. The most common adverse event was thrombocytopenia without clinically significant bleeding. NAV+RUX was tolerable and demonstrated early improvement in disease modification parameters in this difficult-to-treat population. This trial was registered at www.ClinicalTrials.gov as #NCT03222609.