The 21-year old has signed a season-long deal with last season's runners-up who kick-off the 2012/13 campaign with a Grand Final rematch against champions Brisbane Roar on October 7.
Glory head coach Ian Ferguson was thrilled to add Nagai's attacking talents to his squad whilst the player himself was excited about playing with his new teammates.
"I am absolutely delighted to get a left sided player and to get the calibre of Ryo right before the kick-off is fantastic," Ferguson said.
"I had no hesitation in signing him. I knew straight away from his first training session with us that I wanted to keep him."
Nagai added: "I am very happy.
"My first impressions of Australia have been very nice. The people are very kind, and outside of Football, there is also a lot of lovely nature.
"Everyone has been very positive accepting me."