BackgroundPostoperative atrial fibrillation (PoAF) after cardiac surgery has a high incidence of 30%, but its management is controversial. Two strategies are recommended without evidence of a superiority of one against the other: rate control with beta-blocker or rhythm control with amiodarone. Landiolol is a new-generation beta-blocker with fast onset and short half-life. One retrospective, single-center study compared landiolol to amiodarone for PoAF after cardiac surgery with a better hemodynamic stability and a higher rate of reduction to sinus rhythm with landiolol, justifying the need for a multicenter randomized controlled trial. Our aim is to compare landiolol to amiodarone in the setting of PoAF after cardiac surgery with the hypothesis of a higher rate of reduction to sinus rhythm with landiolol during the 48 h after the first episode of POAF.MethodsThe FAAC trial is a multicenter single-blind two parallel-arm randomized study, which planned to include 350 patients with a first episode of PoAF following cardiac surgery. The duration of the study is 2 years. The patients are randomized in two arms: a landiolol group and an amiodarone group.Randomization (Ennov Clinical®) is performed by the anesthesiologist in charge of the patient if PoAF is persistent for at least 30 min after correction of hypovolemia, dyskalemia, and absence of pericardial effusion on a transthoracic echocardiography done at bedside.Our hypothesis is an increase of the percentage of patients in sinus rhythm from 70 to 85% with landiolol in less than 48 h after onset of PoAF (alpha risk = 5%, power = 90%, bilateral test).DiscussionThe FAAC trial was approved by the Ethics Committee of EST III with approval number 19.05.08. The FAAC trial is the first randomized controlled trial comparing landiolol to amiodarone for PoAF after cardiac surgery. In case of higher rate of reduction with landiolol, this beta-blocker could be the drug of choice used in this context as to reduce the need for anticoagulant therapy and reduce the risk of complications of anticoagulant therapy for patients with a first episode of postoperative atrial fibrillation after cardiac surgery.Trial registrationClinicalTrials.gov NCT04223739. Registered on January 10, 2020.Supplementary InformationThe online version contains supplementary material available at 10.1186/s13063-023-07353-6.