Studies on ligamentum flavum hematoma (LFH) have been gradually increasing; however, no study has reported an LFH spreading to the intraspinal and extraspinal spaces. The purpose of this report is to discuss this rare condition and report that extraspinal hematoma can be formed by LFH. The authors present the case of a 78-year-old man presented with right L5 radiculopathy caused by a space-occupying lesion with intraspinal and extraspinal expansions at the L4-L5 vertebral levels demonstrated on MRI. We tentatively diagnosed these lesions as intraspinal and extraspinal hematomas originating from the ligamentum flavum based on the chronological changes seen on MRI and computed tomography-based needle biopsy. After the extirpation of these lesions, the symptoms were relieved. Three months later, the patient could walk without a cane. From the intraoperative findings and pathological examination, we concluded that the extraspinal hematoma in paravertebral muscle was caused by an LFH of unknown etiology. This case report describes the difficulty in diagnosing LFH accompanied by an extraspinal hematoma with wide-spreading expansion and highlights the usefulness of repetitive MRI over time in capturing chronological changes of the hematoma. As far as we know, this is the first study on an LFH accompanied by an extraspinal hematoma in the multifidus.