McKennie, 22, has put pen-to-paper on a one-year loan deal with an option to permanently join at the end of the 2020-21 season.
He landed in Turin on Thursday to undergo a medical ahead of the move to the Italian champions.
McKennie has become the first United States player to sign for Juventus and the club's first signing this summer since Andrea Pirlo replaced Maurizio Sarri as manager earlier this month.
Sources told ESPN on Wednesday that Pirlo was a key figure in getting the deal over the line.
McKennie spent three years in the Bundesliga with Schalke where he made 91 appearances and scored five goals.
McKennie will attempt to win his first major trophy in club football as Juve aim to clinch a 10th consecutive Serie A title.