Background and Objectives: Acute decompensated heart failure (ADHF) leads to significant impairments in exercise capacity and functional outcomes. This study aimed to evaluate the feasibility and effectiveness of integrating early phase I cardiac rehabilitation with a multidisciplinary heart failure post-acute care (HF-PAC) program to improve functional capacity in patients hospitalized for ADHF, assessed by serial cardiopulmonary exercise testing (CPET). Materials and Methods: We conducted a prospective cohort study at a medical center in Taiwan. Patients hospitalized for ADHF between February 2017 and March 2023 who completed inpatient and six-month follow-up CPET were enrolled. The rehabilitation protocol included supervised aerobic and resistance training during hospitalization, followed by outpatient multidisciplinary care. The primary outcome was the change in peak oxygen uptake (peak VO₂) over six months. Results: A total of 90 patients were included (74.4% male, mean age 58.4 ± 14.7 years). Peak VO₂ significantly improved from 11.57 ± 3.33 to 13.99 ± 4.2 mL/kg/min (p < 0.001). Significant improvements were also observed in 6 min walk distance, anaerobic threshold, heart rate recovery, oxygen uptake efficiency slope, and left ventricular ejection fraction. Conclusions: Early integration of phase I cardiac rehabilitation with multidisciplinary HF-PAC is feasible and enhances exercise capacity in patients with ADHF. Serial CPET provides an objective evaluation of functional recovery and may guide rehabilitation strategies in this high-risk population.