Atrial fibrillation (AF) is the most common type of cardiac arrhythmia and is associated with increased short-term mortality in critically ill patients. The creatinine-to-albumin ratio (CAR) is associated with adverse outcomes in various clinical settings. This study investigated the association between CAR and 28-day all-cause mortality and evaluated its contribution to a clinical prediction model. A total of 4501 critically ill patients with AF were identified from the Medical Information Mart for Intensive Care IV database. The optimal cutoff value for CAR was determined using X-tile software, and the patients were categorized into high (>0.5) and low (≤0.5) CAR groups. The Kaplan-Meier analysis demonstrated a significantly lower 28-day survival in patients with elevated CAR. The CAR showed good discriminative ability for 28-day mortality. In multivariate Cox regression analyses, CAR remained independently associated with 28-day mortality, both as a continuous variable (hazard ratio = 1.211, 95% confidence interval = 1.061-1.381, P = .004) and as a categorical variables (>0.5 vs ≤0.5; hazard ratio = 1.377, 95% confidence interval = 1.175-1.615, P < .001). Restricted cubic spline analysis revealed a significant nonlinear dose-response relationship between CAR and mortality risk. Subgroup analyses showed consistent associations across most prespecified subgroups. A multivariate prediction model incorporating CAR was used to construct a nomogram that demonstrated good discrimination, calibration, and clinical utility. Overall, elevated CAR was independently and consistently associated with an increased risk of short-term mortality in critically ill patients with AF. In addition, the nomogram incorporating CAR demonstrated strong predictive performance. Given its routine availability and biological plausibility, CAR may serve as a simple and practical biomarker for early risk stratification and individualized prognostic assessment in this high-risk population.