Objectives: To investigate the incidence of accessory maxillary sinus ostia in superior meatus in patients with clinical and radiological signs of maxillary sinusitis and the association with the development of chronic rhinosinusitis. Design: Retrospective study Setting: Tertiary care hospital Participant: 159 patients examined with paranasal sinus computed tomography scans Main outcome measures: We retrospectively evaluated patients who visited the outpatient department at an academic medical facility between January and April 2020 with a clinical diagnosis of chronic rhinosinusitis. Paranasal sinus axial and coronal computed tomography scans were evaluated for accessory maxillary sinus ostia in superior meatus and confirmed by reconstructed three-dimensional simulation images. The demographic information and incidence of accessory ostia in superior meatus were assessed. The Lund–Mackay score was used to rate chronic rhinosinusitis severity. Analysis of variance was performed to correlate the severity of chronic rhinosinusitis with presenting accessory ostia in superior meatus. Results: Of 159 patients (81 males; 78 females), 41.5% had accessory maxillary sinus ostia in superior meatus. Of these, two-thirds were bilateral and one-third was unilateral. The severity of rhinosinusitis was not correlated with having accessory maxillary sinus ostia in superior meatus, but the presence of accessory ostia was significantly associated with less severe chronic rhinosinusitis (P < 0.001). Conclusions: Accessory maxillary sinus ostia in superior meatus are significantly associated with less severe chronic rhinosinusitis and most cases are bilateral.