Hook was out of contract with Magners League title holders the Ospreys at the end of the season and had been liked to a number of foreign clubs.
Hook's contract will start on July 1 but will not see him feature for the French giants until after the World Cup in New Zealand.
"It has been a tough decision but I'm really excited about the move," said Hook at the announcement in Portmeirion today.
"I've thought about it a lot over the last year and I feel the hard grounds and surroundings will suit me.
"I met the president and the club really appealed to me. I went out there last summer, played there with the Ospreys and it is a great place to play.
"I spoke to Stephen Jones and also Nathan Hines who was out with the Lions and he played for Perpignan. They could not speak highly enough about playing out there. I took all that on board - I respect Stephen as a player and a bloke.
"Everything really appealed to me. There are plenty of reasons and I am looking forward to it.
"The last few months I have known in my head I am going, the last year has been tough. It is a huge decision - it is not just me involved, it is my fiancé Kim and my little boy Harrison as well. Obviously, it is a rugby decision as well. But the positives far outweigh the negatives."
Hook has made 101 appearances for the Ospreys during a successful seven-year stint at the Liberty Stadium.
The 25-year-old, who has played fly-half, centre and full-back, won two Magners League titles and the Anglo-Welsh Cup for the Ospreys.
The 2008 Grand Slam winner has Wales 43 caps to his name ahead of the Six Nations opener against England next Friday and he became a British & Irish Lion on the 2009 tour to South Africa after being called up prior to departure to replace injured countryman Tom Shanklin.