Background Retro-maxillary recess of the sphenoid sinus is an area that is yet to have appropriate description in the literature and usually overlooked. The objective of the current study was to determine the incidence and detailed description of the retromaxillary recess of the sphenoid sinus by multi-slices computed tomography (MSCT) in adults. Paranasal MSCT scan of asymptomatic adults was done for all our subjects to obtain delicate anatomical details of the recess using coronal, axial, and sagittal reformatted capability. Each sphenoid sinus in the CT was revised to find and describe the retro-maxillary recess of the sphenoid sinus regarding its extensions and dimensions. Results This study included 100 adult subjects (200 sphenoid sinuses); retro-maxillary recess was detected in 16.5% (33 sides) of them using MSCT. They were distributed in 20 subjects (20% of all examined sphenoid sinuses). Sixty-five percent of reported recesses were bilateral. Range of antero-posterior diameter (depth) of the recess was 3.9 to 14 mm while its width range was 1.1 to 9 mm. The thickness of the bony septum between the recess and the maxillary sinus ranged from 0.2 to 3.2 mm. Conclusion Current work described in details the retromaxillary recess in CT and its nearby surrounding important structures in all directions. From our results, it is recommended to add the retromaxillary recess of the sphenoid sinus to the preoperative CT checklist particularly in revision cases.