LSm proteins(Sm proteins) are a family of RNA-binding proteins found in virtually every cellular organism. LSm proteins are defined by a characteristic three dimensional structure and their assembly into rings of six or seven individual LSm protein molecules.

The Sm proteins were first discovered as antigens targeted by so called Anti-Sm antibodies in a patient with a form of systemic lupus erythematosus (SLE), a debilitating autoimmune disease. They were named Sm proteins in honor of this patient, Stephanie Smith. Other proteins with very similar structures were subsequently discovered and named LSm proteins. New members of the LSm protein family continue to be identified and reported.