Background: High awareness that cardiovascular disease is the leading cause of death (LCOD) among women is critical to prevention. This study evaluated longitudinal trends in this awareness among women. Methods and Results: Online surveys of US women (≥25 years of age) were conducted in January 2009 and January 2019. Data were weighted to the US population distribution of sociodemographic characteristics. Multivariable logistic regression was used to evaluate knowledge of the LCOD. In 2009, awareness of heart disease as the LCOD was 65%, decreasing to 44% in 2019. In 2019, awareness was greater with older age and increasing education and lower among non-White women and women with hypertension. The 10-year awareness decline was observed in all races/ethnicities and ages except women ≥65 years of age. The greatest declines were among Hispanic women (odds ratio of awareness comparing 2019 to 2009, 0.14 [95% CI, 0.07-0.28]), non-Hispanic Black women (odds ratio, 0.31 [95% CI, 0.19-0.49]), and 25- to 34-year-olds (odds ratio, 0.19 [95% CI, 0.10-0.34]). In 2019, women were more likely than in 2009 to incorrectly identify breast cancer as the LCOD (odds ratio, 2.59 [95% CI, 1.86-3.67]), an association that was greater in younger women. Awareness of heart attack symptoms also declined. Conclusions: Awareness that heart disease is the LCOD among women declined from 2009 to 2019, particularly among Hispanic and non-Hispanic Black women and in younger women (in whom primordial/primary prevention may be most effective). An urgent redoubling of efforts by organizations interested in women's health is required to reverse these trends.