The move had been mooted for most of the summer, but appeared to have come under some threat owing to reported interest from Premier League sides Liverpool and Southampton.
But there was relief all round in Sydney earlier on Wednesday when, speaking at a press conference in Turin, the 37-year-old forward confirmed he was heading down under on a two-year deal.
"I am extremely happy to be joining Sydney FC," said the Juventus icon on his new club`s official website.
"This is a very big moment for me because I want to continue my career in a new part of the world where I can make a major contribution and help grow the game I love.
"The most important thing for me has always been the football and I want to help Sydney FC be successful.
"This is an exciting journey for me and I look forward to joining my new team-mates in Sydney shortly."
Del Piero`s reputation precedes him in Italy, where he is regarded as one of the country`s greatest ever players.
A 91-cap international, he left Juventus at the end of last season with 704 appearances and 289 goals to his name, days after winning a sixth Scudetto with the club.
He also has a Champions League and World Cup winner's medal in his cabinet and his new club are naturally delighted to have added him to their squad ahead of the forthcoming A-League season.
"This signing is a major coup for Sydney FC and marks an historic day for football in this country," said chairman Scott Barlow.
"This is for all Australian football fans.
"Alessandro Del Piero is a global icon of the game and we are honoured he has chosen Sydney FC, ahead of many other options, as the club where he will begin the next chapter of his illustrious football career."
It emerged on Wednesday morning that Liverpool may have been one of the clubs also in the running for Del Piero`s signature.
The Reds are short of striking options after failing to secure a replacement for Andy Carroll before the transfer window shut, but would have been entitled to make a move for free agent Del Piero.
"Liverpool have come in - and they're a big club, there's no denying that," chief executive Tony Pignata told Fox Sports Australia.
"But I think the attraction of Australia might sway us."
The player himself later confirmed as much.
In a lengthy explanation on his official website, he wrote of the commitment Sydney had shown to him, as well as the prospect of heading to Australia, had made his mind up.
"It wasn't an easy decision, I wanted to evaluate with great attention all the possibilities and offers I received in these months. I've made my choice and it's Sydney and the only thing I can say today [Wednesday] is that I can't wait to start," he said.
"Why Sydney? First of all, because they really wanted to have me there. They weren't the only ones, but the project they presented, seemed to be specially tailored for me, it won me over both as an athlete and a person.
"I'm moving to Australia not only to continue my football career and to give my best for my new team, but also to live a new experience that surely will be new for me, enlightening and exciting.
"I was lucky to win everything during my career and to end it in Italy winning once again with the team of my life, Juventus. I was looking for something new, something different, something that wouldn't have things in common with my past, a totally new experience. And I have found it in Sydney."
Sydney`s first A-League fixture is on October 6 away to Wellington, with Del Piero`s first home game expected to come against Newcastle a week later.
He will link-up with former Blackburn midfielder Brett Emerton and ex-Morecambe man Terry McFlynn.