Keller has made a big impression in his debut  A League season with the club providing experience and composure.

The 30-year old missed a month of football due to a knee injury but returned to the startling side last weekend against Perth and made an immediate impression in defence.

Keller says he is pleased to have his playing future settled.

"I am really enjoying playing for Sydney FC," he said. "The club has been fantastic to me and we have a great squad that is really motivated to play top football. I am delighted to be given the opportunity to extend my time in Sydney.

"I love the Sydney lifestyle on the Northern Beaches and look forward to getting to know the city better this summer."

Coach Vitezslav Lavicka says he is delighted to secure Keller's services.

He added: "Stephan is a superb professional for our club. He is a very committed defender who brings so much European experience to our team.

"He provides great on field leadership to his team mates, particularly some of our other defenders. He is a vital part of what we are trying to achieve at Sydney FC."