Rilzabrutinib is a covalent, reversible Bruton tyrosine kinase inhibitor targeting multiple immune thrombocytopenia (ITP)-related mechanisms. The phase 3 LUNA3 study in previously treated adults with persistent/chronic ITP evaluated oral rilzabrutinib 400 mg twice daily (n = 133) vs placebo (n = 69) for 24 weeks. At baseline overall, median age was 47 years, 63% female, 7.7 year median ITP duration, and 28% prior splenectomy. Overall (N = 202), 85 (64%) rilzabrutinib and 22 (32%) placebo patients achieved platelet response (≥50 × 109/L or 30 × 109/L to <50 × 109/L and doubled from baseline) during the first 12 weeks and were eligible to continue. The primary end point, durable platelet response (platelet count ≥50 × 109/L for ≥two-thirds of ≥8 of the last 12 of 24 weeks without rescue therapy), was observed in 31 (23%) rilzabrutinib vs 0 placebo patients (P < .0001). All secondary efficacy end points were significantly superior for rilzabrutinib (P < .05). Median time to first platelet response was 15 days in rilzabrutinib responders. Rilzabrutinib significantly reduced rescue therapy use by 52% (P = .0007) and improved week 25 bleeding scores (P = .0006). Improved physical fatigue was sustained from week 13 (P = .01) through 25 (P = .0003). Treatment-related adverse events were mainly grade 1/2. One rilzabrutinib patient with multiple risk factors had serious treatment-related grade 3 peripheral embolism (lower left leg), and another died from unrelated pneumonia. Rilzabrutinib in patients who failed multiple previous ITP therapies showed rapid and durable platelet response, reduced rescue medication and bleeding, improved physical fatigue, and favorable safety. Trial registration: www.clinicaltrials.gov (#NCT04562766) and www.clinicaltrialsregister.eu (#2020-002063-60).