A 72-year-old man was found to have splenomegaly and an intrasplenic mass in a medical examination. Hamartoma, inflammatory pseudotumor, and sclerosing angiomatoid nodular transformation (SANT) were included as differential diagnoses, but malignant disease could not be ruled out; therefore, laparoscopic splenectomy was performed. A postoperative peripheral blood test showed extreme thrombocytosis and pseudo-hyperkalemia. Thereafter, the patient was diagnosed with essential thrombocythemia and myelofibrosis, with a mutant JAK2-V617F gene. The pathological diagnosis was focal nodular extramedullary hematopoiesis (EMH). EMH sequentially occurs in many hematological disorders, and sometimes presents as hematopoietic masses at several sites; however, presentation as a nodular mass localized in the spleen is rare. It is extremely difficult to diagnose EMH in patients without a past medical history of hematological disorders. However, for a focal nodular mass of the spleen, it is important to determine the treatment strategy with consideration of potential hematological diseases and extramedullary hematopoiesis.