Margaritiferidae is a family of medium-sized freshwater mussels, aquatic bivalve molluscs in the order Unionida. It is the most threatened of all unionid families.
The family is sometimes referred to as the freshwater pearl mussel family, but "freshwater pearl mussel" more often applies to the species Margaritifera margaritifera. The name refers to the thick layer of nacre (mother of pearl) lining the interior of the shell of the species, which enables them to produce pearls.
