IGM may refer to:

• IG Metall, the predominant German metalworkers' union
• IGM Financial, Canada's largest manager and distributor of mutual funds and other managed asset products
• Institut Gaspard Monge, a research institute at the University of Marne la Vallée
• International Grandmaster, a chess title
• Intergalactic medium
• Intragroup medium
• Kingman Airport (Arizona) (IATA Code: IGM)
• Istituto Geografico Militare, the national mapping agency for Italy

Immunoglobulin M, or IgM for short, is a basic antibody that is produced by B cells. IgM is by far the physically largest antibody in the human circulatory system. It is the first antibody to appear in response to initial exposure to an antigen. The spleen, where plasmablasts responsible for antibody production reside, is the major site of specific IgM production.