Harrison has spent the last two seasons on loan at Elland Road making 91 appearances and scoring 10 goals in all competitions.
The 23-year-old, who has been capped by England at Under-21 level, featured in every game for the Whites last season, providing eight assists, as Marcelo Bielsa’s side won the Sky Bet Championship title.
Playing for Manchester United and Liverpool during his youth career, Harrison left the UK and went to college in America and was the number one pick in the MLS SuperDraft in 2016 and joined New York City.
After impressing at Yankee Stadium, Harrison was signed by sister club Manchester City in January 2018, before moving on loan to Middlesbrough.
Since joining Leeds, the winger has been one of the first names on the team-sheet under Bielsa and he will now continue his journey at Elland Road, as the Whites prepare for a return to the Premier League for the first time in 16 years.
He becomes the third signing of the transfer window, following the announcements of Helder Costa and Illan Meslier earlier this month.