Background: This study assessed the eustachian tube (ET) function in adult patients with chronic rhinosinusitis (CRS) using tympanometry and nasal endoscope. Materials and Methods: This was a hospital-based cross-sectional study conducted for 9 months. All participants underwent endoscopic evaluation of the pharyngeal end of their ETs, and the middle ear function was assessed using tympanometry. Endoscopic findings were graded based on a validated mucosal inflammatory endoscopic grading scale. Statistical analysis was carried out using SPSS version 24. Results: A total of 102 CRS patients and age- and sex-matched controls were recruited. Tympanograms suggestive of eustachian tube dysfunction (ETD) types B and C were found among 7.8% and 12.8% of the right and left ears of the CRS groups, respectively. Mucosal inflammatory endoscopic grade that is diagnostic of ETD Grades 3 and 4 was found among 24.5% and 38.2% of the right and left ET of CRS cases, respectively. Conclusion: CRS predisposes patients to anatomical and functional impairment of ET. The relationship between tympanometry and mucosal inflammatory endoscopic grading scale in detecting ETD among CRS patients showed a strong relationship. However, a combination of the two will improve the diagnosis of ETD by evaluating the ET function directly and indirectly.