This study investigated effects of endoscopic sinus surgery (ESS) on eustachian tube dysfunction (ETD) and factors associated with improvement. This was a retrospective study which included 302 patients over 17 years old who underwent ESS between 1 December 2016 and 1 December 2018 for chronic rhinosinusitis with nasal polyps (CRSwNP) (n=126), chronic rhinosinusitis without nasal polyps (CRSsNP) (n=151) or recurrent acute rhinosinusitis (RARS) (n=25). Eustachian Tube Dysfunction Questionnaire 7 (ETDQ-7) is a validated patient-reported outcome measure for assessment of ETD. Preoperative prevalence of ETD, defined as ETDQ-7>14.5, was 59.6% overall; 47.6% in patients with CRSwNP; 64.9% of those with CRSsNP; and 88.0% of those with RARS. Patients with clinically significant ETD had statistically significant decreases in ETDQ-7 scores at two weeks (19.9+8.1,P<0.001), six weeks (17.8+9.3,P<0.001), three months (16.8+8.5,P<0.001) and six months (16.4+7.9,P<0.001) postoperatively. Decreases in ETDQ-7 were also significant when analysed by diagnosis. Through univariate regression analysis, factors identified as being associated with minimally clinically important difference in ETDQ-7 were preoperative ETDQ-7>22, SNOT-22 total score>33, and SNOT-22 ear subscore>6. ETD improvement was associated with higher preoperative ETDQ-7 score, higher preoperative SNOT-22 total or ear subscore scores, posterior ethmoidectomy, and postoperative corticosteroid spray use. Allergic rhinitis did not show an association with ETD improvement. The authors conclude that ETD symptoms likely improve following ESS and remain improved for at least six months, especially in those with higher disease burden, who undergo posterior ethmoidectomy or use postoperative corticosteroid sprays.