Nationwide Building Society is a British mutual financial institution, the seventh-largest cooperative financial institution and the largest building society in the world with over 15 million members. Its headquarters are in Swindon, England.

Nationwide is the result of over a hundred mergers, the most notable of which are its merger with Anglia Building Society in 1987 and Portman Building Society in 2007. It is now the second-largest provider of household savings and mortgages in the UK and has a 7.7% market share of current accounts.