Given the role of comorbid conditions in the pathophysiology of HFpEF, we aimed to identify and rank the importance of comorbid conditions associated with post-hospitalization outcomes of older adults hospitalized for HFpEF. We examined data from 4,605 Medicare beneficiaries hospitalized in 2007–2014 for HFpEF based on ICD-9-CM codes for acute diastolic heart failure (428.31 or 428.33). To identify characteristics with high importance for prediction of mortality, all-cause rehospitalization, rehospitalization for heart failure, and composite outcome of mortality or all-cause rehospitalization up to 1 year, we developed boosted decision tree ensembles for each outcome, separately. For interpretability, we estimated hazard ratios (HRs) and 95% confidence intervals (CI) using Cox proportional hazards models. Age and frailty were the most important characteristics for prediction of mortality. Frailty was the most important characteristic for prediction of rehospitalization, rehospitalization for heart failure, and the composite outcome of mortality or all-cause rehospitalization. In Cox proportional hazards models, a 1-SD higher frailty score (0.1 on theoretical range of 0 to 1) was associated with a HR of 1.27 (1.06 to 1.52) for mortality, 1.16 (1.07 to 1.25) for all-cause rehospitalization, 1.24 (1.14 to 1.35) for HF rehospitalization, and 1.15 (1.07 to 1.25) for the composite outcome of mortality or all-cause rehospitalization. In conclusion, frailty is an important predictor of mortality and rehospitalization in adults aged ≥66 years with HFpEF.