Myxomatous mitral valve disease (MMVD) is characterized by thickening of the valve leaflets and omega-3 (ω-3) supplementation has been associated with modulation of blood pressure (BP) and heart rate, improvement of doppler echocardiographic indices, antiarrhythmic, anti-inflammatory and anti-dislipidemic effects in dogs and humans, although prospective studies of it single use are still absent in the veterinary literature. The objective of this study was to evaluate the influence of ω-3 supplementation in dogs with MMVD. Twenty-nine dogs were followed quarterly for 12 months by clinical evaluation, arterial blood pressure, electrocardiography, doppler echocardiography, thoracic radiography and laboratory tests including inflammatory mediators and cardiac biomarker blood concentrations. The dogs were classified in stages B2 and C, according to the classification proposed by ACVIM 2019. They were randomly assigned to either ω-3 group (ω-3G) or control group (CG). The ingestion of ω-3 reduced the chance of developing arrhythmias by 2.96 times (p = 0.003). The vertebral heart size (VHS) measurements were higher in the control group (p = 0.033). In conclusion, at the dosages used in this study, ω-3 dietary supplementation reduces the volumetric overload, has antiarrhythmic effect and keeps dogs with B2 and C stages of MMVD in milder stages of the disease.