The 2006 Football League Championship play-off final was an association football match which was played on 21 May 2006 at the Millennium Stadium in Cardiff, Wales, between Leeds United and Watford. The match was to determine the third and final team to gain promotion from the Football League Championship, the second tier of English football, to the FA Premiership, the top tier. Reading and Sheffield United, the top two teams of the 2005–06 Football League Championship season, gained automatic promotion to the Premiership, while the clubs placed from third to sixth place in the table took part in play-off semi-finals. Third-placed Watford defeated sixth-placed Crystal Palace in the first semi-final, while fifth-placed Leeds United beat fourth-placed Preston North End. The winners of these semi-finals competed for the final place for the 2006–07 season in the Premiership. Winning the final was estimated to be worth up to £40 million to the successful team.

The final was refereed by Mike Dean and was watched by a crowd of 64,736. It was the last play-off final to be held at the Millennium Stadium, as the new Wembley Stadium was completed in time for the 2007 final. Watford won the match 3–0, with opening goalscorer Jay DeMerit named man of the match. Leeds goalkeeper Neil Sullivan scored an own goal to make the score 2–0 to Watford after 60 minutes, and the final goal was a penalty kick scored by Darius Henderson.

The following season, Leeds's manager Kevin Blackwell was sacked in September, with the club second from bottom, and was replaced by Dennis Wise. The club went into administration the following May and were deducted ten points; they finished the season bottom of the league and they were relegated to the third tier of English football for the first time in the club's history. Watford struggled in the Premiership and were relegated back to the Championship after ending the season bottom of the league, ten points below safety.

Watford finished the regular 2005–06 season in third place in the Football League Championship, the second tier of the English football league system, two places and three points ahead of Leeds United. Both therefore missed out on the two automatic places for promotion to the Premiership and instead took part in the play-offs, along with Preston North End and Crystal Palace, to determine the third promoted team. Watford finished nine points behind Sheffield United (who were promoted in second place) and twenty-five behind league winners Reading.

Leeds had won just one of their final ten league games. Their play-off semi-final opponents were Preston North End with the first leg taking place at Elland Road, Leeds, on 5 May 2006. The match ended 1–1: Preston took the lead with a goal from David Nugent in the 48th minute before Eddie Lewis equalised for Leeds with a free kick in the 74th minute. Billy Davies, the Preston manager, commented after the match: "it is tremendous to come here in front of their biggest crowd of the season and get what is a fantastic result. It is a case of job done." The second leg took place three days later at Deepdale, Preston's home ground. After a goalless first half, a header from Rob Hulse and a low strike from Frazer Richardson saw Leeds secure a 2–0 win on the day and a 3–1 aggregate victory. Leeds were reduced to nine men when both Stephen Crainey and Richard Cresswell were sent off in the second half, with six of their team-mates also shown a yellow card.

Watford's opposition for their play-off semi-final was Crystal Palace, with the first leg held at Selhurst Park on 6 May 2006. After a goalless first half, a strike from Marlon King one minute into the second half opened the scoring for Watford. Ben Foster, on loan from Manchester United, made a fingertip save to deny a header from Crystal Palace's Tony Popovic, before a curling free kick from Ashley Young made it 2–0. Five minutes from full time, Matthew Spring scored to give Watford a 3–0 win. The second leg at Watford's Vicarage Road took place three days later. The home side's manager Aidy Boothroyd was sent to the stand after an altercation with Fitz Hall which resulted in a mass brawl on the pitch. The match finished 0–0, giving Watford a 3–0 aggregate victory and a place in the final.

Leeds United were making their second play-off final appearance: they had lost 2–1 in a replay of the 1987 final to Charlton Athletic after the two legs ended in an aggregate draw. Watford had also previously participated in one play-off final, having beaten Bolton Wanderers 2–0 at the old Wembley Stadium in the 1999 final. During the regular season, the match between the two teams at Vicarage Road in October 2005 was a goalless draw, while Leeds won the return fixture at Elland Road 2–1 the following February. Watford's King was the Championship's leading scorer with 21 goals; his team-mate Darius Henderson was the second-highest scorer with 15. David Healy and Hulse were the top marksmen for Leeds, with 12 each, followed by Robbie Blake on 11. Leeds United had last played in the top tier of English football in the 2003–04 season, when they were relegated after finishing nineteenth in the league. Watford had played in the Championship since being relegated from the Premiership in the 1999–2000 season. Boothroyd was the first-team coach at Leeds United until he left in March 2005 to take the Watford manager's role. In doing so, at the age of 34, he became the youngest manager in the Football League. His playing career had been ended while he was at Peterborough United when he suffered a broken leg in a tackle by Shaun Derry, then playing for Notts County; for the play-off final, Derry was in the starting line-up for Leeds. Boothroyd had been promoted from academy football by the Leeds manager Kevin Blackwell, who himself had experienced failure in the play-off final three years earlier. Then, he was assistant to Neil Warnock whose Sheffield United team lost 3–0 to Wolverhampton Wanderers.

The referee for the match was Mike Dean who was representing the Cheshire Football Association. He had been selected to referee the 2006 FA Cup Final between Liverpool and West Ham United, but was later replaced as he lived in the Wirral. While the Football Association were adamant that they had "complete faith in Dean's refereeing ability, integrity and impartiality", they felt his connection to the Wirral "might lead to comment and debate which could place him under undue additional pressure". It was the last play-off final to be held at the Millennium Stadium, as the new Wembley Stadium was completed in time for the 2007 final. The pitch was in a poor condition following rugby union's Heineken Cup Final which had been hosted at the stadium the previous day. Winning the play-off final was estimated to be worth up to £40 million to the successful team. The chief executive of Watford, Mark Ashton, did not underestimate the impact of promotion: "In my opinion it surpasses the riches of the Champions League – it is the richest football game on the planet".