Background: Endometrial cancer at this time is predominantly being looked at as a disease of postmenopausal population. Increased obesity has been identified as an important risk factor for endometrial cancer. An overall increase incidence of endometrial hyperplasia and endometrial carcinoma in obese premenopausal women has been reported. A close examination of the relationship between trends in endometrial cancer incidence and obesity prevalence in young women will provide important information for prevention and early screening of the disease and its precursors. This study was to assess current trends in endometrial cancer incidence in women ages 20-29 and 30-39 years in relationship to obesity in the US. Methods: We used data on US adult women 20-39 years old from the National Program for Cancer Registries and Surveillance, Epidemiology, and End Results Incidence–U.S. Cancer Statistics 2001–2017 database. This database covered essentially all young female population between 2001 and 2017 in the US (Puerto Rico not included). Incidence was age adjusted to the 2020 U.S. standard population. We also examined the trends in obesity prevalence among females 18-34 years old using data from the National Health and Nutrition Examination Survey (NHANES) III and NHANES 1999-2014. Results: There were 24,446 cases of endometrial cancer among young adult women aged 20-39 years during 2001-2017. Endometrial cancer incidence increased from 0.6 per 100,000 in 2001 to 1.2 per 100,000 in 2017 (APC 3.6, 95% CI 2.9-4.4) among young women 20-29 years old, and increased from 4.6 per 100,000 in 2001 to 7.5 per 100,000 in 2017 (APC 3.0, 95% CI 2.7-3.3) among women 30-39 years old. Obesity prevalence also increased significantly from 1988-2014 among females 18-34 years old. Incidence of endometrial cancer and obesity prevalence were both higher in Hispanics than in other racial/ethnic groups. Conclusions: The significant increasing incidence in endometrial cancer among young adult women is in accordance with the concurrent increasing prevalence in obesity in young girls and women in the US. This indicates that endometrial cancer screening might need to be considered at much earlier age among patients with abnormal bleeding and certain ethnic populations.