Emergency room (ER) visits after surgery can be inconvenient and costly to the patient and the healthcare system. Estimates of the 30-day ER visit rate following ambulatory sinus procedures and their risk factors are largely unknown in the literature. To determine the 30-day postoperative ER visit rate following ambulatory sinus procedures and the causes and risk factors associated with ER visits. This is a retrospective, cohort study using data from the State Ambulatory Surgery and Services Databases (SASD) and the State Emergency Department Databases (SEDD) for California, New York, and Florida in 2019. We identified adult (18 years old) patients with chronic rhinosinusitis who underwent ambulatory sinus procedures from the SASD. Cases were linked to the SEDD to identify ER visits occurring within 30 days after the procedure. Logistic regression models were used to identify patient- and procedure-related risk factors associated with the 30-day postoperative ER visit. Among the 23 239 patients, the 30-day postoperative ER visit rate was 3.9%. The most common reason for ER visit was bleeding (32.7%). A total of 56.9% of the ER visits occurred within the first week. In the multivariate analysis, factors associated with ER visits included Medicare (odds ratio [OR] 1.29 [1.09-1.52], P = .003), Medicaid (OR 2.06 [1.69-2.51], P < .001), self-pay/no insurance (OR 1.44 [1.03-2.00], P = .031), chronic kidney disease/end-stage renal disease (OR 1.63 [1.06-2.51], P = .027), chronic pain/opioid use (OR 2.70 [1.02-7.11], P = .045), and a disposition other than home (OR 12.61 [8.34-19.06], P < .001). The most common reason for ER visit after ambulatory sinus procedures was bleeding. An increased ER visit rate was associated with certain demographic factors and medical comorbidities but not with procedure characteristics. This information can help us identify the patient populations who are at higher risk for ER visits to improve their postoperative recovery.