Background: In older adults (≥65 years) after cardiac surgery, the early post-discharge period often involves functional decline. Early Phase II cardiac rehabilitation (CR) may attenuate this, but the evidence has not yet been systematically synthesized. Objective: To synthesize the effect of early Phase II CR (≤14 days post-discharge) on functioning and health-related quality of life (HRQoL) in older adults after cardiac surgery versus no structured CR or late-initiated CR. Methods: This rapid review will follow Cochrane Rapid Reviews guidance and be reported using PRISMA 2020 and PRISMA-RR. We will search MEDLINE, CINAHL, Cochrane CENTRAL, and trial registries. Randomized and quasi-randomized trials will form the primary evidence base; controlled non-randomized studies will be included for sensitivity analyses. Eligible studies will implement Phase II CR initiated ≤14 days post-discharge and include a predominantly older sample: explicit ≥65 inclusion; or, if not stated, baseline mean/median age ≥70; or age distribution confirming predominance ≥65 (e.g., ≥80%). Primary outcomes are functioning and HRQoL at 30–90 days; secondary outcomes are mortality and readmission at 30–90 days. Screening, full-text assessment, and data extraction will be conducted by two reviewers independently. Risk of bias will be assessed using RoB 2 and ROBINS-I. We will undertake narrative synthesis and, where appropriate, random-effects meta-analysis with sensitivity analyses. Certainty of evidence will be assessed using GRADE for primary outcomes only. Ethics and Dissemination: No primary data will be collected. Findings will be submitted to conferences and peer-reviewed journals to inform implementation and policy decisions regarding early Phase II CR for older adults after cardiac surgery. PROSPERO registration number: CRD420251230368.