Background Splenic rupture by diffuse large B‐cell lymphoma (DLBCL), which usually progresses insidiously, is extremely rare. Case Presentation A 60‐year‐old man presented with paralysis in his lower left extremity. A magnetic resonance imaging suggested transverse myelitis. No lymphadenopathy or organomegaly was noted. Two months after remission, he was referred to the emergency department complaining of presyncope. He was in preshock due to splenic rupture, and underwent laparotomy after attempts of transcatheter arterial embolization. Splenomegaly, hepatomegaly, and disseminated enlarged lymph nodes were observed. Histological examinations of the resected spleen showed DLBCL. He died of multiple organ failure associated with intractable bleeding. His autopsy revealed diffuse systemic invasions of lymphoma cells except for the brain and spinal cord. Microscopically, the spinal cord showed macular incomplete necrosis and histiocytic infiltration, suggestive of hemophagocytic syndrome. Conclusion The progression of DLBCL in our case is drastically rapid. Undiagnosed transverse myelitis preceded the onset.