Coach John Aloisi said he had followed the progress of the former Melbourne Victory player over the years.
The left back was released by Victory in May after featuring in the club's 2018 Grand Final win over Newcastle Jets.
Aloisi, who has coached Nigro in the past, said his latest recruit was known for his versatility and hard-working nature.
"We are pleased to sign another young, talented Australian footballer who will suit our style of play," Aloisi said.
"I've coached Stefan previously and I've tracked his progress for a number of years. He is a welcome addition to our squad.
"We're continuing to work hard as a group and we will be well prepared for our FFA Cup match and the start of the Hyundai A-League season."
Nigro said he was delighted to reunite with Aloisi and play a pivotal role in the club's tilt at a fourth championship.
“(Aloisi) was part of the Victory (youth) coaching staff when I was 18, so I know him quite well and playing under him again was a massive attraction for me,” he said.
He added: “The training facility (at Roar) is top-class, as is the pitch and the set-up behind the scenes. With the squad that’s available, we’ll definitely be up there pushing for the title this year.
"The boys have been really welcoming so I just can’t wait to get started.
“This club is moving forward and have made a lot of good signings already this off-season."
Since progressing through the youth ranks at the Victory, Nigro has made 26 A-League appearances.