Harrison joins Fiorentina on-loan to the end of the season with a permanent option included worth €6m.
Should the Viola trigger the option, Harrison will be handed a deal to 2030.
Fiorentina announced this morning: "ACF Fiorentina announces that it has acquired, on loan with an option to buy, the rights to the sporting performances of the footballer Jack David Harrison from Leeds United FC.
"Harrison, born in Stoke-on-Trent, England, on 20 November 1996, came through the Manchester United Youth Sector and, during his career, in addition to Leeds, also wore the shirts of Middlesbrough, New York City FC and Everton.
"The new Viola player has made 194 appearances between the Premier League and FA Cup, scoring 27 goals and providing 23 assists. Harrison will wear the number 17 shirt ."