Cato made his debut for Saracens in 2007 and finished as the club's top try scorer in 2009. His international experience includes England Saxons, Under-20's and Sevens.
"Northampton is a great club with a superb stadium and set-up," said Cato.
"It's also one of the best clubs in the Premiership in terms of matchday atmosphere. More importantly the Saints have a competitive team and will be exciting to be involved in that.
"Playing in a back three with two England players will be exciting, and the team has a brilliant pack that can give a platform for everyone to play good rugby. I'm looking forward to working with all the coaches at Franklin's Gardens, and I'm sure that with their help I can develop my game and hopefully help the team continue improving as well.
"It is a positive move for me, moving from one big club to another. Saracens has been a good place for me to learn and I'm leaving without any bad memories. My experiences there have certainly helped shape me as a person.
"But I also believe that it is time for me to kick on and hopefully establish myself at the Saints next season."
Director of rugby Jim Mallinder says that Cato is an exciting addition to the Saints squad.
"Noah has shown his abilities and potential for several seasons, both at club level and with England Under-20s and the Saxons," he said. "He can play both on the wing and at full back, and has pace and skill with the ball in hand. He is certainly an excellent prospect who will bring something good to our squad as we continue to build for the future."