Linfield Football Club is a Northern Irish professional football club, based in south Belfast, which plays in the NIFL Premiership – the highest level of the Northern Ireland Football League. The fourth-oldest club on the island of Ireland, Linfield was founded in 1886 by workers at the Ulster Spinning Company's Linfield Mill. Since 1905, the club's home ground has been Windsor Park, which is also the home of the Northern Ireland national team and is the largest football stadium in Northern Ireland. They train at Midgley Park which is beside the stadium. The club's badge displays Windsor Castle, in reference to the ground's namesake.

Historically, Linfield shared a fierce rivalry with Belfast Celtic until Celtic's withdrawal from the league for political reasons in 1949. Since that time the club's main rival has been Glentoran, with the duo known locally as the Big Two. This rivalry traditionally includes a league derby played on Boxing Day each year, which usually attracts Northern Ireland's highest domestic attendance of the season, excluding cup finals. For the 2021–22 season, Linfield's average league home attendance was approximately 2,900, the highest in the division and more than double the league's overall average of around 1,400. The team, nicknamed The Blues, is managed by former Northern Ireland international and the country's all-time record goalscorer, David Healy. Healy was appointed on 14 October 2015 to succeed Warren Feeney, following Feeney's resignation.

Domestically, Linfield has been one of the most successful clubs in the world, holding several national and global records. The club has won 57 league titles, which is more than twice than any other Northern Irish club, and makes Linfield the world's most successful club in terms of national championships won. The club has lifted the Northern Irish Cup 44 times, the second-highest number of national cup wins worldwide. It has also won the League Cup a record twelve times, as well as four all-Ireland cup competitions, among other domestic cups, taking its trophy count to well over 116, one of the most in the world. In the 1921–22 season, Linfield completed an unprecedented clean sweep of all seven available trophies (Septuple) – to date, this is the only recorded instance of a seven-trophy season being achieved in world football. In the modern era, the club won all four available domestic trophies in 2006 to complete a domestic quadruple, and has also won three domestic trebles. The club also holds the world record for the most domestic doubles, with 25. Linfield is one of only three clubs to have completed an Irish League campaign unbeaten, having done so on four occasions.

The club was one of the eight founding members of the Irish League in 1890, won the inaugural league title, and is one of only three clubs to have gone on to compete in every season of the Irish League's top division since; a joint world record for the longest continuous membership of a national league's top division. In European football, the club's best finish is the quarter-finals of the 1966–67 European Cup. Linfield FC has qualified for European competitions over 60 times, making them a frequent representative of Northern Ireland in UEFA tournaments.

The club was founded in March 1886 in an area of south Belfast known as Sandy Row by workers at the Ulster Spinning Company's Linfield Mill. Originally known as Linfield Athletic Club, the team initially played on an area of land located at the back of the mill owned by the company, which was known as the Meadow. Originally, the club had intended to have a rule in place to limit membership to employees of the mill only. However, this idea was quickly scrapped to allow for the strongest team possible, with six of the club's first eleven players being non-employees. Linfield was credited with originating the passing game in Ireland, where a dribbling based approach had been the norm until around 1890, and for three seasons during the club's early years they competed in the English FA Cup. In the 1888–89 qualifying rounds they defeated Ulster and Bolton Wanderers to reach the fourth qualifying round, where they faced Cliftonville. After two draws, they won 7–0 in the second replay, which was played on 25 December 1888. This is notable for being the only FA Cup match ever to be played on Christmas Day.

This victory meant they qualified for the first round proper for the first and only time, where they were drawn to face Nottingham Forest in what would turn out to be a controversial tie. Linfield had earned an impressive 2–2 draw in Nottingham, which meant a replay back in Belfast. In an apparent cup upset, Linfield then defeated Forest 3–1 at the Ulster Cricket Ground in Ballynafeigh, with the large crowd celebrating the fact that Linfield had progressed to the second round. However, a few days later the Belfast Telegraph revealed that the 'replay' had actually been played as nothing more than a friendly. Prior to the match, Linfield had conceded the tie after discovering that they had inadvertently fielded an ineligible player, William Johnston, in the first match. Officials from both clubs had agreed not to reveal this information to the players or the public before the match, in order to play the match as planned. In any case, Linfield would have been unlikely to have played any further part in the cup, as the costs of travelling to Kent to play the second round tie against Chatham at an open ground with no gate income would have been prohibitive. Linfield made their final FA Cup appearance during the 1890–91 first qualifying round, being defeated 5–4 by Nantwich. This was the last season in which Irish clubs entered the competition.

Success on the field meant that the club had to accommodate bigger crowds, bringing about a move to Ulsterville Avenue in 1889. In 1890, Bob Milne signed for the club from the Gordon Highlanders. The Scot would soon become a key member of the team, helping the Blues to lift the Irish Cup at the young age of 20. The club stayed at Ulsterville for five years before housing development on the ground in 1894 meant that the club had to move on once again. Between 1894 and 1897, Linfield played all of their home games at opponents' grounds until the president of the club, Robert Gibson, along with other club members, eventually secured a ground lease at Myrtlefield in the Balmoral area of the city. However, this was another temporary home. The club stayed here until 1905, when they moved into Windsor Park. The club's first silverware at Windsor arrived in the 1906–07 season, with the club lifting both the league title and the County Antrim Shield. This would be the first of a trio of league titles, with the 1907–08 and 1908–09 league titles to follow.

In 1910, team captain Bob Milne left the club with a legacy as one of Linfield's best ever players. He had amassed nine Irish Cups, eight league titles, and had earned 27 international caps for the Ireland national team during his time at the club. Another Scottish player, Marshall McEwan, joined the club in 1911 at the age of 26. He had previously played for English clubs Blackpool, Bolton Wanderers and Chelsea. McEwan is perhaps best remembered for his performance in the 1913 Irish Cup final, described by some fans as the best in years. McEwan retired in 1916, but remained in Belfast and later opened several businesses in the city. In 1915, the Irish League was suspended as a result of the First World War. In its place, a temporary unofficial league known as the Belfast & District League was set up and ran for four seasons until the return of the Irish League in 1919. As this was an unofficial competition, any titles during this time are not counted as Irish League Championships. Linfield won this competition twice, in 1915–16 along with the Irish Cup, and in 1917–18.