"I need to get out of my comfort zone and develop as a player and this move to Northampton will do that," said Dowson.
"I'd heard nothing but positive things about the Saints, which were backed up when I came to look around the club.
"This season Saints have had good cup runs and achieved positive results in the league.
"The coaches are making the club stronger all the time and over the next few years I want to help the Saints push up the Guinness Premiership and hopefully yield some silverware."
Northampton director of rugby Jim Mallinder worked with Dowson, who can play in any of the back row positions, when he took charge of the Saxons for the 2007 Churchill Cup.
Dowson captained the Saxons in their recent international victory over Portugal and will be in the frame for England's summer Test series against Argentina.
Mallinder said: "Phil is an exciting signing for us as we build for next year.
"His leadership skills are excellent, as I know all too well from working with him.
"We want leaders at the club who will represent us well on and off the field and Phil is one such man.
"The back row is a vital area of the game and Phil's know-how, skills and physicality will complement the players we have here already superbly."