Ognenovski hasn't played since he was released by Qatari club Umm-Salal last month and could come straight into the Sky Blues squad for this weekend's huge clash with Adelaide.
It's a huge boost to Sydney's finals hopes after the club missed out on Eddy Bosnar last week, with the experienced defender choosing the Central Coast over the harbour city.
"I'm delighted to be able to sign for Sydney FC," Ognenovski said.
"It is always good when you are wanted by a big club and Sydney FC are an iconic club in Australian football. It will be an honour to pull on the sky blue shirt.
"I'm looking forward to the challenge of playing back in the A-League and I know the standard and the quality of the league.
"Sydney FC is an exciting club and having followed the competition this year, I believe the team on its day can beat anyone.
"I want to play at a level to help the team progress into the finals and hopefully we can do some damage when we get there."
"Sasa has a wealth of experience at international and club level and will bring us extra stability in the back four," Sky Blues coach Frank Farina said.
"He recently helped Australia qualify for the World Cup in Brazil and has also won the Asian Champions League and Asian Player of the Year, so we are bringing in real quality to back up what we already have in that position.
"Sasa demands success and is the type of player and character we want at this club."
Ognenovski is expected to start training with the Sky Blues on Thursday ahead of Saturday's clash with the Reds at Allianz Stadium.
