Infratemporal fossa abscess caused by acute sinusitis, without other predisposing factors, is very rarely reported. We present such a case in a 78-year-old woman who was treated successfully with surgical drainage and antibiotics. She presented with a 10-day history of swelling and pain on the right side of her face. Physical examination revealed swelling on the right mandible area up to the zygomatic arch and the presence of mucopurulent rhinorrhea in the right nasal cavity without abnormal dental findings. Computed tomography revealed a right infratemporal fossa abscess and ipsilateral maxillary sinusitis connected through a defect on the posterior wall of the maxillary sinus. Drainage procedure was performed by navigation-guided endoscopic sinus surgery and intravenous antibiotics were administered. During a four months follow-up period, there was no sign of recurrence. Infratemporal fossa abscess can develop from acute sinusitis without predisposing factors, including dental infection. Surgical drainage along with intravenous antibiotics can lead to successful treatment in such cases.