Atrioventricular septal defect (AVSD) is a complex congenital heart disease characterized by abnormal development of the atrioventricular junction, leading to a single atrioventricular orifice with a common atrioventricular valve. AVSD can be classified into complete, partial and intermediate forms. The defect is uncommon in cats and there is a paucity of literature on AVSD in this species. In the first section of the article the clinical, instrumental and pathological findings of a partial form of AVSD in a domestic shorthair cat are presented. In the second section the embryological pathogenesis of the condition is discussed in the light of the most recent knowledge about AVSD.