The 28-year-old utility player has been trialling with the Hyundai A-League champions since the middle of last month.
He has signed a deal that will take him through to the end of the 2016/17 Hyundai A-League season.
Brisbane Roar Director of Football Ken Stead said Polenz had worked hard in training over the past month.
“Jerome has fitted in well. He adds a lot of experience to the squad and will ensure a healthy competition for places in our team,” Stead said.
Polenz spent two seasons with Western Sydney and was a member of the Wanderers team beaten 2-1 by the Roar in last season’s Hyundai A-League Grand Final.
He comes to the Roar after a short stint with Norwegian First Division club Sarpsborg 08.
Polenz said he was excited to be back in the Hyundai A-League and proud to be with the reigning champions.
“I will bring all my qualities to help the Roar get back to the top of the table again,’’ he said.
Polenz played three times for the German under-19 team and once for their national under-20 team, including appearances at the European Under-19 Championships.
Although Polenz has not played since early November, he has done extensive fitness work in recent weeks and is expected to be considered for squad selection for the Roar’s first game back after the Asian Cup break against Wellington Phoenix at Suncorp Stadium on Monday, February 2 (7pm kick-off).