The 30-year-old's contract at Pittodrie expires at the end of the month and he has penned a two-year deal with the Hornets which starts on July 1.
Severin has spent the past five seasons with the Dons, where he was also captain, and has won 14 caps for Scotland.
Mackay told Watford's official website: "I have kept an eye on Scottish football and Scott's progress.
"He's had five terrific years at Aberdeen and is very well respected. Scott was Aberdeen's captain and that speaks volumes for his character and leadership qualities."