The FTSE 100 Index, (pronounced as footsie) is a share index of the 100 largest companies listed on the London Stock Exchange. The FTSE 100 Index is seen as a barometer of success of the British economy and is the leading share index in Europe. The index is maintained by the FTSE Group, which originated as a joint venture of the Financial Times and the London Stock Exchange.

The full title is "The Financial Times Stock Exchange Index of 100 Leading Shares".

Related indices are the FTSE 250 Index which lists the next largest 250 companies; the FTSE 350 Index which combines the FTSE 100 and 250, and the FT All Share Index which quotes all listed companies.

The constituents of the index are determined quarterly. The largest companies in the FTSE 250 Index are promoted if their market capitalisations would place them in the top 90 firms of the FTSE 100 Index. As of 2004, the threshold is about 2 billion pounds.

The largest 5 companies in the index are: BP; HSBC Holdings; Vodafone Group; Royal Bank of Scotland Group; and GlaxoSmithKline.

The official names of all companies listed on this index usually include the abbreviation plc at the end of their name.