A member of the Sky Blues' Youth League side in 2008-09 and shot stopper for NSW Premier League side Sydney United, Janjetovic joins 2011-12 player of the year Ivan Necevski in Sydney FC's goalkeeping ranks.
"I'm thrilled and excited about joining Sydney FC," Janjetovic said.
"I've worked really hard to get into this position and I'm determined to make the most of it over the next two seasons, and hopefully longer.
"I played a season with the Sydney FC Youth Team and loved it but had an injury that set me back. I've had to wait for my chance and I'd like to thank Sydney United for believing in me and allowing me to get back to the stage where I am able to play for Sydney FC."
Reddy's contract was terminated in April for `serious misconduct'.