The Magpies moved quickly to bring in the former Chelsea and Liverpool boss after taking the decision to sack Steve McClaren on Friday morning.
Benitez has been without a job since been axed by Real Madrid in January just seven months into his Bernabeu reign.
He has penned a three-year deal with Newcastle and takes over a team sitting 19th in the Premier League, one point adrift of safety, following a run of three straight defeats.
The Spaniard’s first game in charge will be against Premier League leaders Leicester City at the King Power Stadium on Monday night.
Newcastle owner Mike Ashley will regard the 55-year-old’s decision to join the club as a major coup given his impressive CV.