Portsmouth Football Club is a professional association football club based in Portsmouth, Hampshire, England. The team is currently competing in the EFL Championship, the second level of the English football league system in the 2024–25 season after winning the 2023–24 EFL League One title. Portsmouth are nicknamed Pompey, a local nickname used by both His Majesty's Naval Base, Portsmouth and the city of Portsmouth. The club adopted blue shirts in 1912 and have mostly used a combination of white shorts and red socks since 1946.

Founded on 5 April 1898, Portsmouth began their early history in the Southern and Western leagues, before being elected into the English Football League and newly formed Third Division in 1920. Portsmouth won two promotions in 1924 and 1927 to reach the First Division, becoming the first football club south of London to do so. After finishing runners-up in two FA Cup finals in 1929 and 1934, Portsmouth won for the first time in 1939. Normal professional football in England was suspended in September 1939 because of the outbreak of World War II, meaning Portsmouth retained the FA Cup trophy for seven consecutive years (the record) until the 1946 FA Cup final.

After World War II, Portsmouth became League Champions of England in two successive 1948–49 and 1949–50 First Division seasons. However, Portsmouth's 32 consecutive years in the First Division ended with relegation to the Second Division in 1959 and was followed by another relegation to the Third Division in 1961. In 1978, Portsmouth were relegated to the Fourth Division (formed in 1958) for the first time before earning three promotions in 1980, 1983 and 1987. After one brief season in the 1987-88 First Division, Portsmouth would remain in the second tier between 1988 until 2003.

Portsmouth became champions of the 2002–03 Football League First Division (restructured as the second tier in 1992) and were promoted to the Premier League. Portsmouth won the FA Cup for a second time in the 2008 FA Cup final. Portsmouth's seven-year stay in the Premier League ended in 2009–10 with relegation, signalling the beginning of a difficult period where the club entered financial administration twice and were relegated three times. After the club was purchased by the Pompey Supporters Trust in 2013, Pompey would begin to recover financially and won the League Two title in 2016–17 and the League One title in 2023–24.

Portsmouth are one of only five English football clubs to have been champions of all four tiers of the professional English football pyramid. Portsmouth's arch-rivals are Southampton, a rivalry based in part on geographic proximity and both cities' respective maritime histories. Portsmouth are also the only professional football club in England which is not located on mainland Great Britain, as the city of Portsmouth and their Fratton Park home football ground are located on Portsea Island.