Background: New postoperative atrial fibrillation (POAF) is one of the most critical and common complications after cardio¬vascular surgery precipitating early and late morbidities. Complete blood count (CBC) is an imperative blood test in clinical practice, routinely used in the examination of cardiovascular diseases. Aim: This systematic review with meta-analysis aimed to determine the strength of evidence for evaluating the association of haematological indices in CBC tests with atrial fibrillation following isolated coronary artery bypass graft (CABG), isolated valvular surgery, or a combination of these treatments. Methods: We conducted a meta-analysis of studies evaluating pre- and postoperative haematological indices in patients with POAF. A comprehensive subgroup analysis was performed to explore potential sources of heterogeneity. Results: A literature search of all major databases retrieved 732 studies. After screening, 22 studies were analysed including a total of 6098 patients. Pooled analysis showed preoperative platelet count (PC) (weighted mean difference [WMD] = –7.07 × 109/L and p < 0.001), preoperative mean platelet volume (MPV) (WMD = 0.53 FL and p < 0.001), preoperative white blood cell count (WBC) (WMD = 0.130 × 109/L and p < 0.001), preoperative neutrophil-to-lymphocyte ratio (NLR) (WMD = 0.33 and p < 0.001), preoperative red blood cell distribution width (RDW) (WMD = 0.36% and p < 0.001), postoperative WBC (WMD = 1.36 × 109/L and p < 0.001), and postoperative NLR (WMD = 0.74 and p < 0.001) as associated factors with POAF. Conclusions: Haematological indices may predict the risk of POAF before surgery. These easily-performed tests should defi¬nitely be taken into account in patients undergoing isolated CABG, valvular surgery, or combined procedures.