Background: Patients with chronic kidney disease (CKD) and type 2 diabetes (T2D) are at risk of atrial fibrillation or flutter (AFF) due to cardiac remodeling and kidney complications. Finerenone, a novel, selective, nonsteroidal mineralocorticoid receptor antagonist, inhibited cardiac remodeling in preclinical models. Objectives: This work aims to examine the effect of finerenone on new-onset AFF and cardiorenal effects by history of AFF in the Finerenone in Reducing Kidney Failure and Disease Progression in Diabetic Kidney Disease (FIDELIO-DKD) study. Methods: Patients with CKD and T2D were randomized (1:1) to finerenone or placebo. Eligible patients had a urine albumin-to-creatinine ratio ≥30 to ≤5,000 mg/g, an estimated glomerular filtration rate (eGFR) ≥25 to <75 ml/min/1.73 m² and received optimized doses of renin–angiotensin system blockade. Effect on new-onset AFF was evaluated as a pre-specified outcome adjudicated by an independent cardiologist committee. The primary composite outcome (time to first onset of kidney failure, a sustained decrease of ≥40% in eGFR from baseline, or death from renal causes) and key secondary outcome (time to first onset of cardiovascular death, nonfatal myocardial infarction, nonfatal stroke, or hospitalization for heart failure) were analyzed by history of AFF. Results: Of 5,674 patients, 461 (8.1%) had a history of AFF. New-onset AFF occurred in 82 (3.2%) patients on finerenone and 117 (4.5%) patients on placebo (hazard ratio: 0.71; 95% confidence interval: 0.53–0.94; p = 0.016). The effect of finerenone on primary and key secondary kidney and cardiovascular outcomes was not significantly impacted by baseline AFF (interaction p value: 0.16 and 0.85, respectively). Conclusions: In patients with CKD and T2D, finerenone reduced the risk of new-onset AFF. The risk of kidney or cardiovascular events was reduced irrespective of history of AFF at baseline. (EudraCT 2015-000990-11 [A randomized, double-blind, placebo-controlled, parallel-group, multicenter, event-driven Phase III study to investigate the efficacy and safety of finerenone, in addition to standard of care, on the progression of kidney disease in subjects with type 2 diabetes mellitus and the clinical diagnosis of diabetic kidney disease]; Efficacy and Safety of Finerenone in Subjects With Type 2 Diabetes Mellitus and Diabetic Kidney Disease [FIDELIO-DKD]; NCT02540993)