Proteus mirabilis (P. mirabilis) meningitis in a neonate is rare, but its recognition is important because the disease progresses rapidly and has poor prognosis. A 4-day-old premature female infant born at 36 weeks and 5 days of gestation presented with symptoms of fever and icteric skin. Initial cerebrospinal fluid findings suggested bacterial meningitis, and treatment with antibiotics was started. On the third day, P. mirabilis growth was found in both blood and cerebrospinal fluid cultures and brain computed tomography revealed normal findings. The patient showed improved clinical symptoms, but brain magnetic resonance imaging on hospital day 18 revealed a brain abscess measuring 4.5×3.1×3.1 cm in the right frontal lobe. Cyst drainage was performed immediately and a catheter was inserted. Follow-up computed tomography revealed a tiny abscess remaining in the right frontal lobe, and follow-up magnetic resonance imaging later demonstrated marked interval regression in the size of the abscess. The patient was discharged on day 57 of hospitalization in good condition. Serial brain imaging should be considered in neonatal cases of P. mirabilis meningitis.