The 2009 Lion 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 Europe's top 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, whose deal is understood to be worth up to £350,000 a season, at the announcement in Portmeirion, north Wales.
"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.
"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 Lion on the 2009 tour to South Africa after being called up prior to departure to replace injured countryman Tom Shanklin.