Janko, 31, arrives from Turkish outfit Trabzonspor on a one-year deal, replacing Italian icon Alessandro Del Piero as Sydney's marquee.
The striker - with 17 goals in 40 international appearances - had previously represented Red Bull Salzburg, Twente and Porto in Europe but he is relishing the opportunity to work alongside new coach Graham Arnold in Australia.
"I am thrilled to be joining Sydney FC and can’t wait to get started," said Janko.
"I have come here to win trophies and play in the Asian Champions League and I am confident that I will achieve this with Sydney FC.
"I have spoken with Scott Barlow and Graham Arnold and I am excited about the future direction of this club. I believe I can contribute a big part to this success."
Janko is the third striker to join Sydney in the off-season, with Bernie Ibini, Shane Smeltz and Alex Brosque following Arnold to the two-time Australian champions.
Arnold said he believes Janko can help Sydney in their quest to win the A-League championship for the first time since 2010.
"He will complement our current strikers well," said Arnold, who replaced Frank Farina at the end of the 2013-14 season.
"We have put in a lot of time and effort to ensure he has the right qualities and right character to fit into our squad, and I'm very happy we have been able to sign him.
"At 6 foot 5 he will cause a lot of problems for opposition defenders and he will give us a different threat up front."
