Mark Wynn, 48, was assaulted after challenging PC Ricki Allsop at Coventry City's stadium in 2011.
An investigation was launched after another officer intervened and later informed senior colleagues about the officer's conduct.
West Midlands Police said PC Allsop was given a final written warning.
Ch Insp Brian Carmichael, from the force's professional standards department, said the settlement had been decided after the officer was brought before a misconduct panel.
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Mr Wynn had attended Coventry's clash with Derby County with his 14-year-old son in September 2011.
Some fans had been protesting about the club's owners and Mr Wynn, who was not involved in the protest, said he challenged PC Allsop over why he was not wearing his badge, and questioned the officers' behaviour towards fans.
Mr Wynn said the officer insulted him before arresting him and taking him to cells in the ground.
He claimed he was then "viciously assaulted" by PC Allsop, who claimed Mr Wynn had been resisting arrest.
"He struck me three times in the face and head with his knee. I was still handcuffed with my hands beneath my back," Mr Wynn said.
A second officer intervened and pulled him off Mr Wynn, before lodging a complaint with the force later that day.
Mr Wynn was subsequently convicted of a public order offence and banned from attending matches.
However, he was not made aware the officer had submitted a written statement about PC Allsop's conduct until after he was successful in getting his conviction and ban overturned.
A spokesman for the Football Supporters' Federation, which provided Mr Wynn with legal help throughout the case, said it was a "concern" that knowledge of the statement was not initially passed on to the Crown Prosecution Service.
"Mark was prosecuted despite the fact the statement fatally undermined PC Allsop's credibility," the spokesman said.
"Had Mark not made a complaint while in custody it is unlikely PC Allsop would ever have been brought to book for what he did."
A football fan who was repeatedly kneed in the head by a police officer after being arrested at a match has received a five-figure compensation settlement.