The 20-year-old joins the Whites from Premier League side Arsenal on a season-long deal, with the club beating off stiff competition to secure his arrival.
Nketiah made his Arsenal debut in September 2017, helping the Gunners beat Bate Borisov in the UEFA Europa League.
A month later he was twice on the score-sheet, helping Arsenal record a 2-1 victory over Norwich City in the Carabao Cup and went on to make his Premier League debut against Brighton & Hove Albion in March 2018.
Last season he made nine senior appearances in all competitions and in his most recent game in May, found the net in a 3-1 win against Burnley.
He has also been capped by England at youth level, from Under-18s-21s.
Nketiah becomes the club’s sixth signing of the summer following the arrivals of Jack Clarke, Helder Costa, Jack Harrison and Illan Meslier and Ben White and he will wear the number 14 shirt, during his time at Elland Road.