He featured in all but one of the Hornets' Championship matches in 2014-15 as they earned automatic promotion to the Premier League, scoring five goals, all of which were free kicks.
"I think England is the best place in Europe to play football, so I was really happy that there was a chance to stay here longer," Tozser, who has signed a two-year contract at Loftus Road, told the club's website.
"I had a fantastic time at Watford, winning promotion last season. Now I'm looking forward to the future with QPR. I will do my best for the team. I feel at my peak, and physically good."
The deal was done too late for Tozser to be registered for tomorrow's match at Huddersfield, so he will not be able to make his debut until after the international break in two weeks.