The pacy 25-year-old was contracted to 2.Bundesliga side Heidenheim but sealed his move on Tuesday to Japan.
Halloran starred for Gold Coast United and Brisbane Roar in the A-League but had been playing since 2013 in Germany's second division.
He has represented Australia on six occasions and was a second-half substitute in all three matches under Ange Postecoglou at the 2014 World Cup.
However, he hasn't been sighted in green and gold since a 2-0 friendly defeat to Belgium later that year - despite playing regularly for Fortuna Dusseldorf and then Heidenheim, who he joined in June 2015.
The last time he was picked for a national squad was as an injury call-up for World Cup qualifiers against Bangladesh and Tajikistan in September 2015.
His move to the J-League will at least see him cross paths with Postecoglou, the coach who handed him his first international chance, after he joined Yokohama F. Marinos following his shock departure from the Socceroos.
Halloran played nine times for Heidenheim this season, making five starts and scoring one goal.