Vitamin B12 deficiency is known to cause a variety of symptoms, including megaloblastic anemia, glossitis, and neuropsychiatric disorders. This case report describes a patient who presented with cognitive decline, psychosis, and seizures due to a severe vitamin B12 deficiency. Following treatment with vitamin supplementation therapy, the patient's condition significantly improved. The literature has also documented similar neuropsychiatric manifestations of vitamin B12 deficiency, highlighting the potential for symptom reversal with prompt and appropriate treatment. Therefore, early diagnosis and treatment of vitamin B12 deficiency are critical to preventing potentially irreversible neurological damage.