The 22-year-old has signed a three-year contract at Watford and joins the club for an undisclosed fee.
Hogg made his debut for Villa against Manchester United last season, but was loaned out to Portsmouth in January.
Villa boss Alex McLeish said: “Everyone at the club wishes Jonathan well and every success in his career.
“Jonathan has a great attitude and we know he will do very well as a player at Watford.”
