Introduction <p>Immune thrombocytopenia (ITP) is an acquired autoimmune disease characterized by increased platelet destruction and thrombocytopenia. Fostamatinib inhibits spleen tyrosine kinase and suppresses platelet destruction. However, clinical evidence of its efficacy in Japanese patients with ITP remains limited.</p> Methods <p>In this study, we retrospectively evaluated the clinical outcomes of fostamatinib, with or without thrombopoietin receptor agonists (TPO-RAs), at a single center in Japan.</p> Results <p>The study included 11 patients. At week 1, all 3 evaluable patients (100.0%) achieved platelet count ≥ 30,000/μL and a twofold increase from baseline; at week 2, 10 of 11 (90.9%) met the criteria. Ultimately, 10 of 11 (90.9%) patients achieved the criteria at least once within 24&#xa0;weeks. All 3 patients using concomitant TPO-RAs (100.0%) achieved the primary endpoint. After fostamatinib initiation, 9 of 11 (81.8%) patients discontinued or reduced their concomitant glucocorticoid (GC) dose. Thrombosis was not observed regardless of concomitant TPO-RA use.</p> Conclusions <p>This study demonstrates the real-world effectiveness and safety of fostamatinib in patients with ITP. Platelet count rose soon after fostamatinib initiation, within 1–2&#xa0;weeks. Fostamatinib may enable reduction or discontinuation of GCs, even in patients with ITP refractory to TPO-RAs.</p> <p><i>Trial registration</i>: UMIN000057096</p>