The transfer was confirmed last night when Borrello put pen to paper on a three-year deal with the Red Devils.
The 21-year-old will join former Roar teammate Jamie Maclaren in the Bundesliga 2 after the A-League joint Golden Boot winner joined Darmstadt 98 earlier this month.
Both players were keen to head overseas at the end of the 2016/17 campaign but it leaves a large hole in Brisbane's attacking stocks.
Kaiserslautern's Sport Director Uwe Stöver said the club was delighted with the signing.
"With Brandon Borello, we were able to sign a young, highly-talented striker who, despite his young age, has already won numerous assignments in the highest Australian league and has been able to gain experience in the national teams of his country since the U20," Stöver told the club's website.
"We are convinced that he will enrich our offensive play with his speed and his physique."
Borello added: I already know that the club has a lot of tradition and very passionate fans. I look forward to getting to know the club, the people and the country.
"I would like to integrate as quickly as possible and will give everything on the pitch to make the fans proud."
Roar coach John Aloisi said Borrello’s transfer proves the club's culture and style of play can produce quality young footballers.
“It is a fantastic opportunity for Brandon, and he goes with our blessing,” Aloisi said.
“We would like to wish him the best of luck in the future, and I know all Brisbane supporters will follow his progress with great interest over the coming seasons.”
The 21-year-old winger made his Roar debut in the Hyundai A-League during the 2013/14 season and went on to play 75 games in Orange, scoring 13 goals.
Borrello emerged as one of the Roar's most potent attackers last season, notching four goals and three assists.
He also burst onto the Asian scene in late January, putting four goals past Global FC in the Roar's first AFC Champions League 2017 qualifier.