Objective: The presence of an ethmomaxillary sinus (EMS) may increase the susceptibility to inflammatory paranasal sinus diseases such as chronic rhinosinusitis (CRS) and cause difficulties in surgical interventions to the paranasal sinuses. Therefore, this study aimed to examine the EMS in patients with and without CRS. Methods: The study included 150 patients (300 sides) diagnosed with CRS by the ear–nose–throat clinic and 151 individuals (302 sides) without CRS. Paranasal sinus computed tomography images were reviewed retrospectively. The presence of an EMS (bilateral or not) and its relationship with age and sex were examined. The severity of CRS was determined with the Lund–Mackay scoring system, and its relationship with EMS was evaluated. Results: The EMS was detected in 7 patients (7/301, 2.32%) and 9 sides (9/602, 1.49%) of 301 patients (602 sides) included. The incidence in the CRS group was 2.6%. Three cases were unilateral, and one was bilateral. The incidence in the control group was 1.98%, two cases were unilateral, and one was bilateral. According to the Lund–Mackay scoring system, the mean CRS severity was 8.62 (±5.47). Its severity was 5.25 (±3.94) in the EMS group and 8.71 (±5.48) in the non-EMS group. Conclusion: No statistically significant difference was found between the groups with and without CRS in terms of the presence of EMS (p = 0.723). No evidence reveals that EMS increased the severity of CRS.