Background: Breast cancer in young women is diagnosed at more advanced stages and has a less favorable prognosis. We investigated trends in breast cancer incidence by race/ethnicity, hormone receptor status, and tumor stage in women aged 20-49 years over the past 25 years, as well as the impact of period and cohort effects on these trends. Methods: We used data from Surveillance, Epidemiology, and End Results (SEER) 13 registries for 1993-2002 and SEER 18 registries for 2003-2017. We calculated age-standardized incidence rates and annual percent change (APC), and stratified by race/ethnicity, hormone receptor status (estrogen receptor [ER] and progesterone receptor [PR]), and tumor stage (I-IV) for 222,424 women aged 20-49 years with a primary invasive breast cancer. We performed age-period-cohort analysis (presented as incidence rate ratios [IRR]) to investigate the effects of age, period, and cohort on incidence trends using the 1948 cohort and 1993-1997 period as the reference groups, respectively. Results: Between 2010-2017, invasive breast cancer incidence increased (APC = 0.67%, 95%CI: 0.32 to 1.03) among women aged 20-49 years, after being stable from 1993-2010. There were differences by race over the 25-year period (1993-2017). We observed significant increases in incidence among non-Hispanic White (NHW) (APC = 0.25%, 95%CI: 0.16 to 0.34), non-Hispanic Asia/Pacific Islander (NHAPI) (APC = 0.58%, 95%CI: 0.34 to 0.82), and Hispanic women (APC = 0.59%, 95%CI: 0.34 to 0.83), but not among non-Hispanic black (NHB) women (APC = 0.14%, 95%CI: -0.06 to 0.34). Incidence increased for ER+ tumors but decreased for ER- tumors: ER+/PR+ (APC = 2.39%, 95%CI: 2.20 to 2.58), ER+/PR- (APC = 1.46%, 95%CI: 1.05 to 1.87), ER-/PR+ (APC = -6.33%, 95%CI: -7.31 to -5.33), and ER-/PR- (APC = -0.70%, 95%CI: -1.09 to -0.32). The decrease in ER-/PR- tumors appeared largely driven by decreases among HNW women. Incidence for stages I (APC = 0.31, 95%CI: 0.07 to 0.55), II (APC = 0.99, 95%CI: 0.82 to 1.16), and IV (APC = 2.88, 95%CI: 2.37 to 3.39) tumors increased while that for stage III tumors decreased (APC = 0.81%, 95%CI: -1.04 to -0.59). Both the cohort and period effects impacted incidence, with the cohort effect almost 10 times larger than the period effect. Age-specific relative risk by birth cohort initially decreased between 1948 and 1958 but steadily increased from 1958 to 1993. Breast cancer incidence was higher among women born in the 1988 (IRR = 1.17, 95%CI: 1.07 to 1.28) and 1993 (IRR = 1.22, 95%CI: 0.99 to 1.51) cohorts than for those born in 1948 cohort. Conclusions: Breast cancer incidence is increasing among young women, mainly driven by increases in ER+ tumors. Prevention efforts need to focus on how we can address factors driving the increase in ER+ tumors and also learn from what has worked for decreasing ER- tumors.