β-Thalassemia is associated with several abnormalities of the innate immune system. Neutrophils in particular are defective, predisposing patients to life-threatening bacterial infections. The molecular and cellular mechanisms involved in impaired neutrophil function remain incompletely defined. We used the Hbb^th3/+ β-thalassemia mouse and hemoglobin E (HbE)/β-thalassemia patients to investigate dysregulated neutrophil activity. Mature neutrophils from Hbb^th3/+ mice displayed a significant reduction in chemotaxis, opsonophagocytosis, and production of reactive oxygen species, closely mimicking the defective immune functions observed in β-thalassemia patients. In Hbb^th3/+ mice, the expression of neutrophil CXCR2, CD11b, and reduced NAD phosphate oxidase components (p22phox, p67phox, and gp91phox) were significantly reduced. Morphological analysis of Hbb^th3/+ neutrophils showed that a large percentage of mature phenotype neutrophils (Ly6G^hiLy6C^low) appeared as band form cells, and a striking expansion of immature (Ly6G^lowLy6C^low) hyposegmented neutrophils, consisting mainly of myelocytes and metamyelocytes, was noted. Intriguingly, expression of an essential mediator of neutrophil terminal differentiation, the ets transcription factor PU.1, was significantly decreased in Hbb^th3/+ neutrophils. In addition, in vivo infection with Streptococcus pneumoniae failed to induce PU.1 expression or upregulate neutrophil effector functions in Hbb^th3/+ mice. Similar changes to neutrophil morphology and PU.1 expression were observed in splenectomized and nonsplenectomized HbE/β-thalassemia patients. This study provides a mechanistic insight into defective neutrophil maturation in β-thalassemia patients, which contributes to deficiencies in neutrophil effector functions.