The 27-year-old has signed a deal until June 2025 and follows the arrival of Hassane Kamara from Nice.
The Brazilian, called up to the senior national team for the first time in 2019, made 147 appearance for Udinese.
The addition of Samir, who is left-footed, will provide more defensive cover for Hornets boss Claudio Ranieri as he aims to keep the club in the Premier League.
Watford currently sit 17th in the Premier League table, two points clear of the relegation zone, but have played a match more than third-bottom Burnley.
The Hornets have lost six straight games since beating Manchester United and travel to Ranieri’s former club Leicester on Saturday in the third round of the FA Cup.