The right back, who has also played for Dutch clubs Twente and Feyenoord since moving to England with Middlesbrough as a teenager, underwent a thorough four-day medical before signing for the Australian champions.
"I've been playing overseas for a long time now and have never had the chance to play in the A-League, so this is an extremely exciting opportunity for me," said Wilkshire, who won 80 caps for his country.
"I've played UEFA Champions League football in Europe and the thought of competing with the biggest clubs in Asia in the AFC Champions League is another huge test. I can't wait to get started."
Wilkshire was part of the 'golden generation' of Socceroos who played in Europe's top leagues and took Australia to the 2006 and 2010 World Cups.
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A stalwart of the Australia defence for a decade from 2004, Wilkshire previously played under Graham Arnold when the Sydney FC boss was in charge of the national team.
"I've never put a player through the type of strenuous physical testing I’ve had our fitness team put Luke through this week," Arnold said.
"We needed him to prove his fitness and at 35, he has shown he is in top physical condition.
"I had a great working relationship with Luke in the national team and I look forward to doing so again."
Sydney FC open their title defence in a re-run of last season's championship final against Melbourne Victory on Oct. 7.
(Reporting by Nick Mulvenney, editing by Ian Ransom)
