Everitt, one of the most prolific goalkickers in Premiership history, has agreed a three-year deal and will join Saints next season.
Everitt, 30, said: "This is quite a commitment for me, as it will probably be my last long-term contract.
"I look forward to the challenge ahead and being part of something special as Saints move forward."
Northampton head coach Paul Grayson added: "Barry is a proven performer over a number of years, and he has great experience in the Premiership.
"His kicking game obviously stands out as exceptional. However, he is a very capable footballer and distributor, and there is a lot more he has to offer."