IMPORTANCE: The optimal timing and approach for initiating cardiac rehabilitation (CR) in critically ill patients during the acute phase of acute decompensated heart failure (ADHF) remains uncertain. OBJECTIVE: To evaluate the effects of CR on physical function and rehospitalization for critically ill patients with ADHF admitted to the cardiac intensive care unit (CICU). DESIGN, SETTING, AND PARTICIPANTS: In this single-center, single-blind randomized clinical trial conducted in China, critically ill patients with severe ADHF admitted to the CICU were recruited between March 26, 2021, and September 1, 2022. All patients were followed up for 6 months, and investigators were blinded to the group assignment. INTERVENTIONS: After short-term therapy, participants were randomized 1:1 to an early progressive and personalized CR program for patients with ADHF (AHF-CR program) that was administered exclusively during the patients' CICU stay or to usual care. MAIN OUTCOMES AND MEASURES: The primary outcomes were Short Physical Performance Battery (SPPB) score at hospital discharge and 6-month all-cause rehospitalization rates. These outcomes were analyzed using an intention-to-treat approach including all patients after randomization. The Perme Intensive Care Unit Mobility (PERME) score was incorporated as an exploratory outcome during analysis to assess mobility status in critically ill patients. RESULTS: This study included 120 patients (mean [SD] age, 68.6 [12.3] years; 80 [66.7%] male). At randomization, pulmonary crackles were observed in 49 patients in the control group (81.7%) and 43 patients in the intervention group (71.7%). Additionally, 62 patients (51.7%) had an arterial partial pressure of oxygen to fraction of inspired oxygen ratio below 300 mm Hg. A total of 40 patients (33.3%) received intravenous vasoactive medications, and 87 (72.5%) received intravenous loop diuretics. The median difference in SPPB scores between groups was 1.0 (95% CI, 0-2.0; P = .16), which was not significant. Six-month rehospitalization rates were comparable between the control and intervention groups (16 [26.6%] vs 17 [28.3%]; hazard ratio, 1.00 [95% CI, 0.51-1.99]; P = .99). Exploratory analysis revealed that the intervention group had higher PERME scores, with a median between-group difference of 2.76 (95% CI, 0.77-4.74; adjusted P = .04). CONCLUSIONS AND RELEVANCE: In this randomized clinical trial of critically ill patients with ADHF, the AHF-CR program did not significantly improve SPPB scores or rehospitalization rates. However, it may offer potential physical benefits, including enhanced mobility. TRIAL REGISTRATION: Chinese Clinical Trial Registry Identifier: ChiCTR2100050151.