A building society is a financial institution owned by its members as a mutual organization. Building societies offer banking and related financial services, especially savings and mortgage lending. Building societies exist in the United Kingdom, Australia and New Zealand, and used to exist in Ireland and several Commonwealth countries. They are similar to credit unions in organisation, though few enforce a common bond. However, rather than promoting thrift and offering unsecured and business loans, the purpose of a building society is to provide home mortgages to members. Borrowers and depositors are society members, setting policy and appointing directors on a one-member, one-vote basis. Building societies often provide other retail banking services, such as current accounts, credit cards and personal loans. The term "building society" first arose in the 19th century in Great Britain from cooperative savings groups.

In the United Kingdom, building societies actively compete with banks for most consumer banking services, especially mortgage lending and savings accounts, and regulations permit up to half of their lending to be funded by debt to non-members, allowing societies to access wholesale bond and money markets to fund mortgages. The world's largest building society is Britain's Nationwide Building Society. Further, in Australia, building societies also compete with retail banks and offer the full range of banking services to consumers.