Portsmouth Football Club is an English professional association football club in Portsmouth, Hampshire. The team compete in EFL League One, the third tier of the English football league system. The club was founded on 5 April 1898 and home matches are played at Fratton Park, their original home ground which was first opened on 15 August 1899. Portsmouth are also known as Pompey, the local nickname for both the city of Portsmouth and HMNB Portsmouth. Uniquely, Portsmouth is the only club in English professional football which is not located on the mainland of Great Britain, as the club and the city of Portsmouth are both built on Portsea Island instead.

Portsmouth have been the top tier League Champions of England twice; in the consecutive 1948–49 and 1949–50 seasons. Portsmouth have won the FA Cup twice; in 1939 and 2008, the FA Charity Shield once in 1949 and the EFL Trophy once in 2019. They have also won the second tier title once in 2002–03, the third tier three times in 1923–24 (South), 1961–62, 1982–83 and the fourth tier once in 2016–17. By winning the League Two title in 2017, Portsmouth became only the fifth English football club to win all four tiers of current English professional football. In addition, Portsmouth are also one of only two English football clubs (along with Wolverhampton Wanderers) to have been champions of five professional divisions including the former regional Football League Third Division South championship in the 1923–24 season. Before their election into the national English Football League in 1920, Portsmouth were also champions of the regional Southern Football League in 1901–02 and 1919–20. The team were also champions of the Western Football League in 1900–01, 1901–02 and 1902–03.

After earning promotion from the Second Division in the 1926–27 season, Portsmouth became the first football club south of London to reach the First Division, the top tier of professional English football. They also became the first club in the Football League to gain promotion through three professional divisions; from the Third Division (1920–21 to 1923–24 (South)), to the Second Division (1924–25 to 1926–27) and reaching the First Division for the 1927–28 season. Portsmouth remained in the First Division for a further 32 years until relegation to the Second Division at the end of the 1958–59 season. Their most recent period in the top tier was between 2003 and 2010, when they also participated in the 2008–09 UEFA Cup, Portsmouth's only appearance in European competition. The club's fortunes declined between 2010 and 2013 when the club entered financial administration twice and were relegated three times, dropping to the fourth tier and their lowest point since the 1979–80 season.

The club were saved in 2013 from High Court liquidation after being bought out by the fan-owned Pompey Supporters Trust (PST) on 10 April 2013. This made Portsmouth the largest fan-owned football club in England until 3 August 2017, when the PST sold it to The Tornante Company, an investment company owned by former Disney CEO Michael Eisner.

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