Forestieri was found guilty of using racist language towards Mansfield Town defender Krystian Pearce in the July 2018 pre-season friendly by the governing body's independent regulatory commission.
The FA, in a detailed, lengthy report of the hearing, stated that Forestieri was alleged to have called Pearce a "n*****" during the confrontation. Forestieri denied that claim, but he admitted he using other abusive Spanish words towards Pearce.
The flashpoint sparked a mass brawl on the pitch at the One Call Stadium and Forestieri was later fined £25,000 and handed a three-game suspension for misconduct relating to the fight.
Despite being acquitted last March at Mansfield Magistrates Court of racially abusing Pearce, the FA charged Forestieri in June, deeming there to be enough evidence to find the Argentinian-born former Italy under-21 striker in breach of its own regulations.
Forestieri contested the commission's verdict but his six-match suspension has been upheld, meaning he will sit-out their fixtures with Huddersfield Town, Fulham, Everton (cup), Middlesbrough, Hull City and Wigan Athletic.
The 29-year-old will next be available for first-team duty after the second international break when the Owls travel to Cardiff City on Friday, October 18. Forestieri has also been fined £25,000.
An FA statement read: "Fernando Forestieri will be unavailable for Sheffield Wednesday’s next six matches after an appeal against a proven breach of FA Rule E3(2) was dismissed by an Appeal Board.
"Words used by the striker during a friendly fixture against Mansfield Town FC on 24 July 2018 were deemed to have breached Rule E3(1) as they were abusive and/or insulting and constituted an ‘aggravated breach’, which is defined in Rule E3(2), as they included reference to ethnic origin and/or colour and/or race.
"He was also fined £25,000, warned as to his future conduct and must attend face-to-face FA education.
"The Appeal Board’s written reasons will be made available via TheFA.com in due course."
Forestieri has made on start and three substitute appearances this season.
A statement on the club's official website read: “We further acknowledge that Fernando will be unavailable for the next six first team fixtures.
“The club and the player remain extremely disappointed with the findings of the regulatory commission and the appeal board when Fernando was acquitted in relation to the same incident in a court of law earlier this year.
“Both parties will be making no further comment at this time.”