Tumor lysis syndrome is caused by the massive lysis of tumor cells. It frequently occurs in rapidly proliferating malignancies and less often after solid tumor treatment. Spontaneous tumor lysis syndrome is extremely rare and only occurs in bulky, advanced, and metastatic tumors. Electrolyte abnormalities in tumor lysis syndrome could be life-threatening without prompt recognition and treatment. In this case, we present a spontaneous tumor lysis syndrome in a 22-year-old man with primitive neuroectodermal tumor. When managing bulky, advanced solid tumors, especially with liver metastasis, cautious observation should be sought because of the possibility of spontaneous tumor lysis syndrome.