The 26-year-old returns to England after previously starring for Bournemouth, helping the Cherries secure Premier League promotion in 2022 before joining Udinese a year later.
Born in London, Zemura began his career with Charlton and can also operate in midfield, providing Watford with valuable versatility.
The Zimbabwe international has earned 22 caps for his country and will now strengthen the Hornets’ squad for the upcoming campaign.
“Zemura is a player with key experience in the Championship with Bournemouth,” said Group Technical Director Gian Luca Nani.
“He is a full-back of excellent technique and quality, capable of contributing very effectively to the attacking phase. His experience in Serie A has further improved him defensively.
“He brings experience, quality and a winning mentality. We are really happy to have him at Watford.”