To assess multimorbidity burden and its association with clinical outcomes in adults with heart failure (HF) according to sex, age, and HF type. Retrospective cohort study. Five healthcare delivery systems across the United States. Adults with HF (N=114,553). We characterized participants with respect to the presence of 26 chronic conditions categorized into quartiles based on overall burden of comorbidity (<5, 5-6, 7-8, ?9). Outcomes included all-cause death and hospitalization for HF or any cause. Multivariable Cox regression was used to evaluate the adjusted association between categorized burden of multimorbidity burden and outcomes. Individuals with more morbidities were more likely to die than those with fewer then 5 morbidities (5-6 morbidities: adjusted hazard ratio (aHR)=1.27 (95% confidence interval (CI)=1.24-1.31; 7-8 morbidities: aHR=1.52, 95% CI=1.48-1.57; ?9 morbidities: aHR=1.92, 95% CI=1.86-1.99). There was a graded, higher adjusted rate of any-cause hospitalization associated with 5 or 6 (aHR=1.28, 95% CI=1.25-1.30), 7 or 8 (aHR=1.47, 95% CI=1.44-1.50), or 9 or more (aHR=1.77, 95% CI=1.73-1.82) morbidities (vs <5). Similar findings were observed for HF-specific hospitalization in those with 5 or 6 (aHR=1.22, 95% CI=1.19-1.26), 7 or 8 (aHR=1.39, 95% CI=1.34-1.44), or 9 or more (aHR 1.68, 95% CI=1.61-1.74) morbidities (vs <5). Consistent findings were seen according to sex, age group, and HF type (preserved, reduced, borderline HF), in the association between categorical burden of multimorbidity and outcomes especially prominent in individuals younger than 65. After adjustment, higher levels of multimorbidity predicted worse HF outcomes and may be an important consideration in strategies to improve clinical and person-centered outcomes. J Am Geriatr Soc 66:2305-2313, 2018.