The 33-year-old, who has 77 England caps and was in the British & Irish Lions squad for their 2017 1-1 series draw against world champions New Zealand, is keen to remain in Red Rose coach Eddie Jones' plans ahead of next year's World Cup so needed to join an English club.
Haskell was told he can leave Wasps after two spells with the Ricoh Arena outfit which totalled 12 years, but he is now also delighted to get an opportunity to help the Saints improve on a disappointing 2017-18 campaign.
"I want to continue my international career for as long as possible," he told the Saints website.
"The only way I can do that is by putting my best foot forward in club rugby and helping Saints fight their way back into contention for silverware.
"I'm really excited and humbled to be joining a giant of English rugby. This is a club with a proud history and a set of passionate and loyal supporters who always make their presence felt."