Northern Rock, formerly the Northern Rock Building Society, was a British bank. Based at Regent Centre in Newcastle upon Tyne, United Kingdom, Northern Rock was originally a building society. It demutualised and became Northern Rock bank in 1997, when it floated on the London Stock Exchange with the ticker symbol NRK.

During the early 2000s the company borrowed substantially to fund mortgages, with the aim of ambitious growth, and also donated large amounts to charitable purposes and communities directly and through sponsorships. Due to the 2008 financial crisis, it was unable to produce income as expected from its loans, and was at risk of being unable to repay the amounts it had borrowed. The news that the bank had approached the government for support with its liquidity led within 24 hours to a public lack of confidence and concerns that savings were at risk, and the bank failed following a bank run as people rushed to withdraw their savings. It was the first British bank in 150 years to fail due to a bank run.

Unable to find a commercial buyer or secure the further government support needed, it was taken into public ownership in 2008, as an alternative to insolvency. By that point the government had extended liquidity support of tens of billions of pounds to Northern Rock. An inquiry concluded that the board had failed to properly protect the bank from the risks inherent in its strategy, or to restrain the executive directors where required, therefore although the bank had sufficient assets, it had become vulnerable.

The branch operations were eventually returned to private ownership when the branches and other retail operations were acquired by Virgin Group in 2012, being rebranded as Virgin Money the same year. The mortgage book of higher risk assets was renamed Northern Rock (Asset Management) and later "NRAM plc", and remained in public ownership until it was sold to Cerberus Capital Management in 2016.

As of May 2024^[update] the Northern Rock Shareholder Action Group are continuing their campaign to obtain compensation for the shares that were taken over by the Government when the bank was nationalised during the 2008 financial crisis.

Northern Rock Building Society was formed in 1965 by the merger of two North East of England building societies, both of which were based in Newcastle-upon-Tyne: the Northern Counties Permanent Building Society (established in 1850) and the Rock Building Society (established in 1865). During the following 30 years, Northern Rock expanded through the acquisition of 53 smaller building societies, most notably the North of England Building Society in 1994.

Along with many other UK building societies in the 1990s, Northern Rock chose to demutualise as a building society and float on the stock exchange as a bank. Throughout this period an argument against demutualisation was that the assets of a mutual society were built up by its members throughout its history, not just by current members, and that demutualisation was a betrayal of the community that the societies were created to serve. Northern Rock chose to address these concerns by establishing the Northern Rock Foundation, which funded community-based projects. At its Stock Exchange flotation on 1 October 1997 (when it converted from a building society to a bank), Northern Rock distributed shares to members with savings accounts and mortgage loans; the flotation share price was £4.52.

In 2000, it was promoted to the FTSE 100 Index. After the 2007 crisis, it was demoted to the FTSE 250 in December of that year.