The 69-year-old replaces Xisco Munoz at the helm, after the Spaniard was sacked on Sunday after less than 10 months in the job.
The Italian left Serie A side Sampdoria in the summer, and his appointment at Watford is the 20th different club he’s managed at.
Over the course of his 35-year managerial career, Ranieri has managed all across Europe, including in Italy, Spain and Franc. He also had a spell coaching the Greek national team.
Ranieri began his managerial career in 1986 at Vigor Lamezia in Italy and in his native country, has coached leading clubs Juventus, Roma, Napoli, Inter Milan and Fiorentina.
In Spain, Ranieri managed Atlético Madrid and Valencia twice either side of a successful stint at Chelsea.
Over his career, Ranieri has won eight trophies, including the 1998-99 Italian Cup and 2004-05 Super Cup with Valencia as well as the Premier League title in 2016.
He was sacked just a season after his remarkable Premier League title victory at Leicester, with the club just one point above the relegation zone with 13 matches left.
Watford’s next Premier League game is at home against Liverpool on Saturday, October 16.
Ranieri will be the 15th permanent manager Watford have had since 2012. He will be the 14th different manager as Quique Sanchez Flores has been there twice.
The Pozzo family, headed by Giampaolo Pozzo, took over Watford in 2012, and not been afraid to change mangers to achieve success.
Now under the leadership of Giampaolo’s son Gino, the club established itself for a number of years in the division, but was relegated in 2020 before regaining Premier League status earlier this year.