The midfielder joined Spurs from Montpellier last summer and returns to France for a fee of around £6m after just one season in the Premier League.
Watford were also in for the 24-year-old, who started just four league games in England and picked up as many yellow cards.
He said: “I am delighted and very proud to become a member of the Paris Saint-Germain family.
“I wasn’t necessarily planning to leave the Premier League after one year but when a club with the dimension of Paris Saint-Germain calls you, it makes your choice a lot easier!
“That’s the moment when you can experience the power of attraction surrounding this club. I will now focus on my preparation.”
PSG chairman Nasser Al-Khelaifi said: “France is going to rediscover a player who was full of promise when he was playing in Ligue 1.”
The French midfielder is the latest high profile departure from White Hart Lane this summer as chairman Daniel Levy looks to trim some of the high-earning players who have failed to command a regular first-team place under Mauricio Pochettino.
Brazilian midfielder Paulinho joined Chinese club Guanzhou Evergrande in a deal worth £9.8m, Etienne Capoue has moved to Watford for around £6m, Lewis Holtby made his loan move to Hamburg permanent and Younes Kaboul has joined Sunderland just 12 months after being named as club captain.