The 24-year-old has agreed a three-year deal and is Leeds' second summer signing following the arrival of Amari Miller from Birmingham City.
Harrison was part of the squad that ended Leeds’ 16-year wait to return to the Premier League in July 2020 and combined eight goals with eight assists in 36 appearances last season as Marcelo Bielsa’s side finished ninth.
The midfielder spent seven years at Manchester United’s Academy before moving to the United States at the age of 14.
Harrison made his professional debut for New York City in 2016 and joined Man City in January 2018 before spending time out on loan at Middlesbrough and Leeds.
He will hope to feature in Leeds’ first match of the season when they travel to Man Utd on Saturday 14 August.