The 33-year-old has signed a one-year contract with the Saints after spending 12 seasons with their English Premiership rivals.
Haskell said that he now has his sights set on a place in next year's Rugby World Cup squad, as well as winning silverware with his new club.
"I want to continue my international career for as long as possible," Haskell 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."
Haskell has won 77 caps with the England national team.