The veteran forward has brought to an end reports linking him with a move to Premier League side Liverpool by signing a two-year contract with Sydney.
Del Piero said: "This is a special day for me. I'm delighted to announce that a few hours ago I reached an agreement with Sydney FC and for the next two seasons by future will be in Australia.
"I'm looking forward to what will be a sensational adventure.
"I want to thank all of the clubs that have looked at me, those with which we have spoken and their officials.
"It has been a long process but I needed to make the right choice. I didn't expect so many clubs to look at me. I won't reveal their names, but we touched all of the world's continents.
"Now this adventure can begin."
Sydney chief executive Tony Pignata added: "This signing, of a legend, is a sensational thing not only for us, but for the whole of Australian football."