Del Piero, who ended his 19-year association with Juve when leaving on a free transfer at the end of last season, has signed a two-year deal worth an annual 1.6 million euros to complete the biggest transfer in the history of Australian football.
Liverpool's failure to sign a new striker on transfer deadline day had forced them to attempt to sign Del Piero as back-up to Luis Suarez and Fabio Borini, but the forward shunned the chance to move to Anfield at such a late stage.
Del Piero, who scored 208 goals in 513 appearances for Juve, will explain his decision in a press conference in Turin on Wednesday afternoon.
"It is a special day for me," Del Piero said. "I'm happy to announce that I have signed agreement with Sydney for the next two years.
"I'm happy to begin this adventure and I want to thank all the teams that I have talked to. There was no better option than this, thank you Sydney FC and everyone in Australia.
"A lot of clubs contacted me, Italian teams too and I thank all of them. Today my new adventure starts and my emotions are mixed.
"I thank Sydney for this opportunity and his fans that already adopted me. I will discover a fantastic city and a fantastic country. I am grateful to my family for being with me in every moment.
"I will continue to cheer Juventus. In Australia I will be able to deepen my passion for rugby. Above all I'm here to score a lot of goals and to try to improve Australian football with my experiences. I hope Australian people love football."
Del Piero will play with the same Number 10 he donned at Juventus, Tony Pignata, Sydney's general manager, said in Turin.
"This signing is a major coup for Sydney FC and marks a historic day for football in this country," club chairman Scott Barlow said in a statement. "This is for all Australian football fans".
Football's popularity in Australia trails behind the rival football codes of rugby league and Australian Rules in the congested domestic market place.
With most of Australia's best players plying their trade abroad, the top flight A-League has suffered from lack of star quality since it was launched for the 2005-6 season.
Former Manchester United striker Dwight Yorke, England striker Robbie Fowler and Socceroos John Aloisi, Harry Kewell and Brett Emerton were among the best known players to have graced the league before Del Piero's signing.
The presence of Kewell, now departed, and Emerton, who will play alongside Del Piero for Sydney FC, helped push attendances past the average mark of 10,000 last season.
Del Piero's arrival could not have been more timely time for the A-League.
The Football Federation of Australia (FFA)'s TV deal, which comes with the sweetener of the hugely popular Socceroo matches, expires at the end of the season and the arrival of such a high profile player can only help negotiations.
The new season begins in early October with twice champions Sydney FC taking on New Zealand's Wellington Phoenix across the Tasman Sea in their opener on October 6.