Formerly of Parma and Sampdoria among others, the 28 year-old played 23 matches in Serie B for Perugia last season.
Rossi also represented Italy at under-17, under-19, under-20 and under-21 level.
He was once infamously punched by Zlatan Ibrahimovic while playing for Bari against AC Milan.
Oddly, it wasn't the only time he was involved in such an incident, he was also punched by Inter Milan's Christian Chivu during a match.
Phoenix coach Merrick has been working on getting the defender to Wellington for over three months.
“I’m sure the time spent on signing Marco will be well worth it,” Merrick said.
“It was vitally important that we replaced Ben Sigmund with an experienced player because you need two experienced players at the heart of defence.
“As a centre back it is very much about reading the play and starting possession from the back and Marco has the experience to do that.
“Centre back is all about judging the offside line and I believe Marco and Dura (Andrew Durante) will form a good combination.
“No other country in the world has developed defenders like Italy and our young players like Dylan Fox can learn a lot from someone like Marco.”
Rossi’s capture won’t be the end of the club’s signings with Merrick looking for a fullback, defensive midfielder and a back-up striker.
Phoenix general manager David Dome said Rossi’s signing was a key part in the club having a competitive squad for the 2016-2017 season.
“The club is committed to Ernie’s playing strategy of an exciting brand of football, hence the signing of Kosta Barbarouses and Gui Finkler with significant investment,” Dome said.
“We will take our time in making further acquisitions but they will be completed before the season begins.
“If we put together the right squad it will help us achieve our objectives of increased attendance and television viewing figures.”
Rossi arrived in Wellington in driving rain and bitterly cold temperatures after leaving Italy in 35 degree heat.
“It is a bit different that is for sure,” a smiling Rossi said.
Asked why he had decided to join the Phoenix Rossi said he felt it was time for a change and wanted to try something different.
“I spoke to (former Socceroo) Vince Grella and he said it would be good experience to come down here,”.
“I also talked to (Socceroo goalkeeper) Mark Birighitti who told me a lot about the A-League and was really enthusiastic about it.
“I took my time before making the decision because going to the other side of the world is a big change.”
Born and raised in Parma, Rossi said he had been training hard for the last three weeks as he realised he would be a bit behind the Phoenix players when he arrived.
