Seeley won the British Superstock Championship in 2009 and British Supersport Championship in 2011, while he finished sixth in the British Superbike Championship in 2010 after fighting for the title in the six-rider Showdown.
He also won North West 200 races in his four years with the outfit.
"I've had a great time working with Philip and Hector [Neill], all the boys in the team, Suzuki GB and our various sponsors. They took me to two British titles and I learned a lot about all aspects of the sport during this time," he said.
"For that I'd like to thank them and Suzuki, but everything comes to an end at some point and I admire Philip for being honest with me so early after the season has finished. It allows me time to find a new position.
"It's a small industry and as Philip says, who knows, I could be back on a Suzuki at some point before I hang the leathers up."
In a disappointing 2012 BSB season he finished down in 13th.
Philip Neill said: "After a tough year in BSB, Alastair has decided that he would like to return to the Supersport class – a decision that I fully support.
"Unfortunately we are unable to offer Alastair a position in Supersport at this time, as our main focus will again be in British Superbike with Tyco and Suzuki in 2013.
"It's never an enjoyable day when you have to let a rider go, especially when he has brought success to your team."
- Sports & Recreation