Nagai made 17 appearances for Glory last term after joining on loan in September, scoring two goals and helping Perth to a second successive finals appearance.
 
He established himself as a firm fan favourite with the Glory faithful, and coach Alistair Edwards his hoping to see more of Nagai's pace and trickery in the 2013/14 campaign.
 
"I'm thrilled that we've managed to bring Ryo back to the club for the whole of next season," Edwards said. "Ryo finished last season extremely well and became very much a crowd favourite.
 
"From the time myself and Gareth (Naven) came in, he improved with each game.
 
"He's very versatile and can play wide on the right or left and also down the middle and he's so well thought of here at Glory. He's become one of the family and everyone loves him.
 
"During my discussions with people within the WA football community, I was asked so many times whether Ryo was coming back, so it's great to be able to say that he is and he'll be with us right through until the end of the season."
 
Nagai will join his teammates for pre-season training on Friday.