McKennie, 22, secured a move from German side Schalke to the Italian champions last month on a one-year loan deal with an option to permanently join at the end of the 2020-21 season.
"It is a great honour to be the first American player in the history of Juventus," he told a news conference on Monday.
"Having the opportunity to wear this shirt, representing my country and bringing young Americans closer to football makes me proud.
"Juventus is an international club, it is famous all over the world. In the United States, shops sell the jerseys. If I can contribute to further growth of the brand, I will be delighted."
"There was no need to convince me to come to Juventus, because being here is a dream that has become a reality," he added.
"Playing with Cristiano [Ronaldo], seeing how he moves and how he works will bring me great benefits. The idea of coming to play here, in a club with a great history, immediately thrilled me."
McKennie becomes the club's first signing since Andrea Pirlo replaced Maurizio Sarri as Juve manager last month.
He becomes the latest U.S. international to join one of Europe's top clubs with Christian Pulisic at Chelsea while Tyler Adams and Gio Reyna continue to impress at Bundesliga clubs RB Leipzig and Borussia Dortmund respectively.
McKennie was one of the first footballers to protest against George Floyd's death in May and said he will continue to be vocal in his opinions for change.
"In Germany, I committed myself to the Black Lives Matter movement," he added.
"We want to take as much action as possible in order to change the situation; I had full support in Germany and it's great that I can carry on what I believe in here in Italy as well, as the club immediately showed support in my commitment."
McKennie could feature this weekend as Juventus get their Serie A campaign underway against Sampdoria.