Colchester United Football Club is a professional association football club based in the city of Colchester, Essex, England. The team competes in EFL League Two, the fourth level of the English football league system.

Founded in 1937, the club spent its early years playing in the Southern Football League until they were elected to the Football League in 1950. Between 1950 and 1990, Colchester spent their time between the Third Division and Fourth Division, during which time they produced one of their most memorable results, a 3–2 victory in the fifth round of the FA Cup over Don Revie's Leeds United in 1971.

Colchester United were relegated to the Football Conference in 1990 following a decline in the late 1980s, but won the Conference title in 1992 to make a swift return to League football. They achieved promotion to the Second Division in 1998 following a 1–0 win against Torquay United in the play-off final. The club were again promoted in 2006, achieving second place in League One. The following season, they achieved their highest league finish in club history, ending the season 10th in the Championship ahead of East Anglian rivals Ipswich Town, Norwich City and also Essex rivals Southend United, despite having the division's lowest attendance. The club returned to League One in 2008 following relegation from the Championship and then made a return to the fourth tier for the first time in 18 years in 2016.

Colchester United play their home games at Colchester Community Stadium in Colchester. They relocated to the stadium in 2008 when they moved away from Layer Road, their home stadium for 71 years.

Until 1937, Colchester Town were Colchester's main club and were the original tenants of Layer Road. Colchester Town joined the Eastern Counties League in 1935, but their poor performances in the league convinced supporters that the club should turn professional, much like nearby Ipswich Town. With club officials against the idea of turning professional, a new professional club was formed in March 1937, Colchester United, which would also play at Layer Road. United joined the Southern Football League as crowds for Town matches dwindled. In December 1937, Colchester United formed a reserve team, signing many of Town's players. As a result of this and Town struggling with £300 debts, Colchester Town folded the same month.

The club were Southern League champions in 1939 prior to the Second World War. Following the war, in 1947–48, the U's produced one of the most notable FA Cup runs by a non-league side, defeating fellow non-leaguers Banbury Spencer in the first round, before beating Football League clubs Wrexham, Huddersfield Town and Bradford Park Avenue. They finally fell to Blackpool in the fifth round. This set them in good stead for potential election to the Football League.

Colchester United were elected to the Football League in 1950 on the back of their second Southern League Cup win and ending the 1949–50 season second to Merthyr Tydfil on goal average alone. They spent eleven years in the Third Division South and Third Division following the league's reorganisation, with a best finish of third place in 1957, just one point behind rivals Ipswich Town and Torquay United.

The club suffered their first relegation in 1961 as they finished 23rd in the Third Division, but didn't have to wait long until their first Football League promotion, spending just one season in the Fourth Division as they ended the season second to Millwall by just one point. This trend continued over the next two decades as they were relegated to the Fourth Division in 1965 and promoted to the Third Division in 1966, then relegated in 1968 and promoted in 1974, relegated in 1976 and promoted in 1977 before a final relegation to the Fourth Division in 1981.