Objectives Sjogren syndrome (SS) is an autoimmune disease with oral manifestations. SS may result in hormonal imbalance and immune dysfunction, and its occurrence could be related to comorbidities, including endometriosis. The prevalence rate of endometriosis in women with chronic pelvic pain is greater than 33%, and in SS patients, the prevalence rate is 6.3%. Given the limited number of available studies, we sought to determine whether patients with endometriosis have a higher risk of SS. Study Design We performed a prospective cohort study of females with newly diagnosed endometriosis from 1999 to 2013 from the Longitudinal Health Insurance Database (LHID). Data on age and history of underlying comorbidities were retrieved from the LHID. Data on the use of corticosteroids and nonsteroidal anti-inflammatory drugs were also compared. A total of 16,214 patients were identified to have endometriosis. Among them, 15,949 patients received a diagnosis of SS after the index date and were included for analysis; 31,880 (7.10%) of 448,953 eligible patients without a history of endometriosis were recruited as age-matched controls. A Cox proportional hazard model was developed with endometriosis as the a priori dichotomous variable, with the aim to estimate the risk of SS in patients with endometriosis. A cumulative probability model was adopted to assess the time-dependent effect of endometriosis on the development of SS, implying the casual link of the association. Sensitivity analysis was conducted to further confirm the risk of SS in patients with endometriosis. Results The mean ± standard deviation age (38.8 ± 8.8 years) was the same for patients with or without endometriosis (P = .406). The Cox proportional hazard model analysis showed that patients with endometriosis were more likely to have SS than patients without endometriosis (adjusted hazard ratio [HR] = 1.5; 95% confidence interval [CI] = 1.27–1.77; P Conclusions Our findings suggest an association between endometriosis and a higher risk of SS. Endometriosis may be a risk factor for, or share a common cause with, SS. This is the first study with an extended follow-up period addressing both the timing and development of SS in patients with endometriosis.