The 74-year-old returns to management just eight months after leaving his post at Crystal Palace and has signed a short-term contract.
The club said on social media: "We are delighted to confirm the appointment of Roy Hodgson as the club's manager. Welcome to Watford, Roy!"
The former England, Liverpool, and Inter Milan boss will now be tasked with saving the Hornets from relegation to the Championship.
Their situation is perilous right now as they sit second from bottom of the Premier League table with 14 points from 20 games.
Watford are just two points from safety right now and have games in hand on some of their rivals.
But they will know Newcastle United will be making new signings this transfer window, giving them a harder task.
Watford's Italian owners, the Pozzo family, know Hodgson having worked with him at Udinese in June 2001.
On that occasion, he lasted just 17 games, winning seven, drawing five, and losing five, before being sacked in December of the same year.
Long-term assistant Ray Lewington, once a Hornets manager himself, will join him at Vicarage Road
The Hornets sacked Ranieri after just 13 Premier League games in charge.
A 3-0 defeat at home to relegation rivals Norwich sealed his fate having won just two league fixtures, against Everton and Man United.