Breast cancer and heart failure (HF) are frequently interconnected due to shared risk factors and the cardiotoxicity of breast cancer treatment. However, the association between HF and hospital outcomes among breast cancer patients has not been studied. This study examined the association between HF and hospital outcomes among hospitalized patients with breast cancer. This cross-sectional study using the 2015���2018 Healthcare Cost and Utilization Project-National Inpatient Sample data included hospitalized women who were aged 18 years or older and had a primary diagnosis code for breast cancer. Logistic regression, negative binomial regression, and generalized linear models with log-link and gamma distribution were used to assess the associations of HF with in-hospital mortality, length of stay (LOS), and hospital costs. Among 17,335 hospitalized patients with breast cancer, 4.2% (n = 1,021) had HF. Compared to breast cancer patients without HF, those with HF were more likely to die during hospitalization (odd ratio =1.65, 95% CI =1.27-2.16, p < 0.001), stay in the hospital longer (incidence rate ratio =1.22, 95% CI =1.15-1.30, p HF has a substantial negative impact on health outcomes among hospitalized breast cancer patients. Breast cancer and HF are often considered separate medical conditions, but promoting effective management of comorbid-HF in breast cancer patients may help to improve hospital outcomes in this population.