The 25-year-old has been at Bundesliga side Freiburg since 2018, but has only managed eight appearances for the first team in that time.
His first season with Freiburg was curtailed by a knee injury and he spent most of his second season on the bench.
Borrello will be hoping to re-establish himself in a league he performed well in 2017-18, where he played 19 matches for FC Kaiserslautern, scoring three goals and creating eight assists.
“I'm really looking forward to playing for Fortuna in the coming season. I know the 2. Bundesliga and know that Fortuna is a large and traditional club - when those responsible contacted my advisor and me, I wanted to play for the club straight away," Borrello said via Fortuna's website.
"I'm happy to be here and look forward to getting started."
Fortuna's Sports Director, Uwe Klein, praised Borrello's mentality and versatility.
“Brandon Borrello will improve our attacking game with his mentality and aggressiveness," Klein said.
"He can be used on both wings and offers our coaching team even more options in attack. We are very happy that Brandon will join us in the coming season."
