A 44-year-old man was referred in our hospital for asthenia, digestive disorders, faintness with fall at home. A computed tomography scan revealed a severe ruptured spleen, which immediately led to splenectomy. At this time, the blood count showed high leukocytosis (290 × 10 9/L) and anemia (8 g/dL) without thrombocytopenia (170 × 10 9/L). Blood smear examination revealed 96% of small blast cells with a high nuclear/cytoplasmic ratio and irregular nucleus (panel A; May-Grünwald-Giemsa stain, original magnification ×500). Flow cytometry detected CD45 dim/cytCD3 +/sCD3 −/CD34 −/TdT +/CD1a +/CD2 +/CD5 +/CD7 +/CD4 −/CD8 + T lymphoblasts (panel B). Surprisingly, the white blood cell count decreased drastically without treatment to 3 × 10 9/L 2 days later. The blast proportion also decreased to 1% with a concomitant high cell lysis rate confirmed by elevated lactate dehydrogenase (7830 U/L), suggesting that the spleen was the main source of blood blast cell dissemination. Histology confirmed massive infiltration of the spleen by cortical lymphoblastic T cells (panel Ci, hematoxylin and eosin stain [HES]; panel Cii, immunohistochemistry [IHC]). Due to an unsuccessful aspiration, bone marrow infiltration was not assessed. Molecular analysis demonstrated an unfavorable prognosis with T-cell acute lymphoblastic leukemia (T-ALL) classifier (NOTCH/FBXW7/RAS wild-type, PTEN mutation and deletion). Eleven days after splenectomy, induction chemotherapy by Group of Research on Adult ALL (GRAALL) protocol was started, which allowed a complete remission.This case reveals a very unusual primary splenic T-lymphoblastic leukemia/lymphoma with a spontaneous blast cell decrease before initial treatment, highlighting a very rare blast clearance following a ruptured pathologic spleen.