Benitez signed a three-year contract to take over in the north-east, while Fabio Pecchia, Francisco de Míguel Moreno and Antonio Gómez Pérez join him as his backroom staff.
The former Real Madrid coach will be tasked with saving the club from relegation to the Championship, after a disastrous Premier League campaign leaves them in serious danger of the drop.
Benitez, who was sacked from the Santiago Bernabeu in January after just half a season in the Spanish capital, was confirmed as McClaren's replacement a matter of hours after the ex-England boss was relieved of his duties.
"I have the pleasure to confirm I have committed to a legendary English club, with the massive challenge of remaining part of the Premier League," the manager said, according to a Newcastle statement.
"It will be a challenge not just for me and my staff but for the players, the Club and the fans.
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"All of us must push together in the same direction and with the same target in mind. This is the reason why I'm going to ask for your total support to successfully complete this task.
"Personally, it means my return to the Premier League, closer to my home and my family. I can't be happier.
"C'mon Toon Army! The club and I need your total involvement!"
Newcastle managing director Lee Charnley, meanwhile, highlighted Rafa's track record and pledged to back the new man in his bid to avoid relegation.
"I am very pleased to welcome Rafa Benitez to the club. In Rafa we have, without doubt, secured the services of one of Europe's top managers," he stated.
"He has managed some of the most successful teams at the very highest level of the game and we are proud to now have him as our manager.
"Our sole focus now is to give our full support to Rafa, his coaching team and the players in order to secure our status in the Premier League."