SkyBet Championship: Watford's boss resigns as side sit second in table!
The 57-year-old Italian, who only took up the role in December, says he has "gone as far as I can" in the role.
He added: "Whoever will be in charge in the future will have everything ready for him to deliver the success that everyone at the club would wish for."
The Hornets are just one point off top spot in the Championship after Saturday's 4-2 win over Huddersfield.
A statement from the club read: "Watford FC wishes to thank Beppe Sannino for all that he has achieved while with the club and we wish him well for his future in football."
Sannino had never previously worked outside of Italy when he was appointed as Gianfranco Zola's replacement by Watford's owners, the Pozzo family, in December.
He took over with the club lying 13th in the Championship but guided them to a strong start this season, with four wins from five league games.
Despite success on the pitch, Sannino's future had been the subject of speculation amid reports some of the players were unhappy with his management style.
Asked about his future after Saturday's win over Huddersfield, he said: "I don't think I'm in a position to ask the club about my future because the results are talking clearly.
"I work always; this is my job, I love my job, I love Watford, I love everything about Watford."
However, just a day later he quit his post to leave a third vacancy in the Championship, with Leeds United and Huddersfield also searching for new managers.
"I took over a very talented squad when I arrived - but they were boys in many ways," Sannino added.
"I'm proud of what we have achieved and I have no doubt that the squad now, who look very much like men who know exactly what they must do, are very capable of getting promotion from this tough Championship league.
"I know I have given everything to Watford; I have given my heart and soul to the job. However I feel I have gone as far as I can so, for the benefit of me and the club, I feel the time is right for me to move on."