The Portman Building Society was a mutual building society in the United Kingdom, providing mortgages and savings accounts to consumers and offering loans to commercial enterprises. Its head office was in Bournemouth and its administration centre in Wolverhampton. Portman merged with the Nationwide Building Society in August 2007, at which time it was the third largest building society in the UK and the largest regional building society in the south of England, with 154 branches and assets exceeding £15 billion.

A building society is a financial institution owned by its members as a mutual organization, which offers banking and related financial services, especially savings and mortgage lending. They exist in the United Kingdom, Australia and New Zealand, and formerly in Ireland and several Commonwealth countries, including South Africa as mutual banks. They are similar to credit unions, but 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.