The Financial Times Stock Exchange 100 Index, also called the FTSE 100 Index, FTSE 100, FTSE, or, informally, the "Footsie" /ˈfʊtsiː/, is a share index of the 100 companies listed on the London Stock Exchange with the highest market capitalization. It is seen as a gauge of prosperity for businesses regulated by UK company law. The index is maintained by the FTSE Group, a subsidiary of the London Stock Exchange Group.

The index began on 3 January 1984 at the base level of 1000; the highest closing value reached to date is 7103.98 (with an intra-day high of 7122.74), on 27 April 2015, the previous peak having been over 15 years previously, on the last trading day of 1999, during the dot-com bubble. After falling during the financial crisis of 2007-2010 to below 3500 in March 2009, the index recovered to a peak of 6091.33 on 8 February 2011, fell under the 5000 mark on the morning of 23 September 2011, but reached its record high on the market close of 27 April 2015, more than doubling in value from the crash in 2009. It then went into a bear market, closing at 5537 on 11 February 2016.