Racial disparities in timely diagnosis and treatment of surgical conditions exist; however, it is poorly understood whether there are hospital structural measures or patient-level characteristics that modify this phenomenon. To assess whether patient race and ethnicity are associated with delayed appendicitis diagnosis and postoperative 30-day hospital use and whether there are patient- or systems-level factors that modify this association. This population-based, retrospective cohort study used data from the Healthcare Cost and Utilization Project's state inpatient and emergency department (ED) databases from 4 states (Florida, Maryland, New York, and Wisconsin) for patients aged 18 to 64 years who underwent appendectomy from January 7, 2016, to December 1, 2017. Data were analyzed from January 1, 2016, to December 31, 2017. Delayed diagnosis of appendicitis, defined as an initial ED presentation with an abdominal diagnosis other than appendicitis followed by re-presentation within a week for appendectomy. A mixed-effects multivariable Poisson regression model was used to estimate the association of delayed diagnosis of appendicitis with race and ethnicity while controlling for patient and hospital variables. A second mixed-effects multivariable Poisson regression model quantified the association of delayed diagnosis of appendicitis with postoperative 30-day hospital use. Of 80 312 patients who received an appendectomy during the study period (median age, 38 years [IQR, 27-50 years]; 50.8% female), 2013 (2.5%) experienced delayed diagnosis. In the entire cohort, 2.9% of patients were Asian or Pacific Islander, 18.8% were Hispanic, 10.9% were non-Hispanic Black, 60.8% were non-Hispanic White, and 6.6% were other race and ethnicity; most were privately insured (60.2%). Non-Hispanic Black patients had a 1.41 (95% CI, 1.21-1.63) times higher adjusted rate of delayed diagnosis compared with non-Hispanic White patients. Patients at hospitals with a more than 50% Black or Hispanic population had a 0.73 (95% CI, 0.59-0.91) decreased adjusted rate of delayed appendicitis diagnosis compared with hospitals with a less than 25% Black or Hispanic population. Conversely, patients at hospitals with more than 50% of discharges of Medicaid patients had a 3.51 (95% CI, 1.69-7.28) higher adjusted rate of delayed diagnosis compared with hospitals with less than 10% of discharges of Medicaid patients. Additional factors associated with delayed diagnosis included female sex, higher levels of patient comorbidity, and living in a low-income zip code. Delayed diagnosis was associated with a 1.38 (95% CI, 1.36-1.61) increased adjusted rate of postoperative 30-day hospital use. In this cohort study, non-Hispanic Black patients had higher rates of delayed appendicitis diagnosis and 30-day hospital use than White patients. Patients presenting to hospitals with a greater than 50% Black and Hispanic population were less likely to experience delayed diagnosis, suggesting that seeking care at a hospital that serves a diverse patient population may help mitigate the increased rate of delayed diagnosis observed for non-Hispanic Black patients.