Objectives <p>This study aimed to assess the frequency of the accessory maxillary ostium (AMO) and its association with the mucous retention pseudocyst (MRC) of the maxillary sinus in an Iranian population.</p> Materials and Methods <p>This cross-sectional study was conducted on 107 CBCT scans retrieved from the archives of the Oral Radiology Department of School of Dentistry, Guilan University of Medical Sciences, Rasht, Iran. The coronal sections were evaluated by an oral radiologist for presence/absence of AMO and MRC. Age and gender of patients were also recorded. Data were analyzed by the Chi-square test and Fisher’s exact test (alpha = 0.05).</p> Results <p>CBCT scans of 47 males (43.9%) and 60 females (56.1%) with a mean age of 46.64 ± 13.82 years (range 18 to 84 years) were evaluated. The frequency of AMO was 57.9% in the study population. The AMO frequency had no significant association with gender (<i>P</i> = 0.486), or age (<i>P</i> = 0.447). The frequency of MRC was 20.6% on the CBCT scans. The MRC frequency was significantly higher in males (<i>P</i> = 0.037) but had no significant association with age (<i>P</i> = 0.657). The AMO frequency had no significant association with MRC (<i>P</i> = 0.115).</p> Conclusion <p>AMO had a frequency of 57.9% in the study population, and had no significant association with MRC of the maxillary sinus.</p> Clinical Relevance <p>A large mucous retention pseudocyst in the maxillary sinus can fill the entire sinus cavity, potentially leading to complications such as inflammation and nasal obstruction, which can affecting treatment and may require treatment.</p>