Galectin-3 (Gal-3) is a lectin associated with fibrosis and inflammation, and increased circulating concentrations are considered a risk factor for atrial fibrillation (AF) in humans. This retrospective study aimed to evaluate the serum concentration of Gal-3 in dogs with cardiac disease, both with and without AF. Dogs with AF associated with congenital and acquired heart diseases were selected, while clinically healthy dogs and dogs with heart diseases but without AF served as controls. We statistically compared the serum concentration of Gal-3, assessed using a commercial canine-specific ELISA kit, among healthy dogs and dogs with heart disease with and without AF. Additionally, associations between Gal-3 and clinical and echocardiographic variables were evaluated. A total of 80 dogs were included, of which 17/80 (21.2%) were clinically healthy and 63/80 (78.8%) had heart disease, with 30/63 (47.6%) having AF. No significant difference in Gal-3 concentration was found between healthy dogs (3.90 ± 0.38 ng/mL) and dogs with heart disease, either with or without AF (3.45 ± 0.28 ng/mL, P=0.226 and 4.46 ± 0.27 ng/mL, P=0.286, respectively). Gal-3 showed a significant positive correlation with age (r=0.46, P < 0.001). The results of this study suggest that Gal-3 does not have predictive value for the development of AF in dogs, but it is associated with advanced age.