McKennie has made nearly 100 appearances for Juventus since joining in 2020, scoring 13 goals and providing five assists, and has won the 2020 Italian Super Cup and the 2021 Coppa Italia with the team.
He has 41 caps for the US national team and helped them reach the last 16 in the World Cup in Qatar last year.
McKennie will join fellow US internationals Tyler Adams and Brenden Aaronson at Leeds, who are coached by American Jesse Marsch.
Leeds have also signed defender Max Wober and forward Georginio Rutter during the January transfer window.
Leeds currently sit 15th in the Premier League standings, just one point above the relegation zone halfway through the season. Their next match is against Nottingham Forest on Sunday.