The 20-year-old has been a fixture in multiple German youth teams, and was a virtual ever-present in Germany under-19s' European Championship win last summer.
Holthaus, who was born in Hamm, made his 2. Bundesliga debut back in May 2013 for Bochum - when he was just 18.
Since then, he made a total of nine second division appearances for the Ruhr Valley club and a further two in the DFB-Pokal.
Now at Fortuna, Holthaus has signed a three-year deal. However, the clubs agreed not to make the transfer fee public.
Holthaus and Kramer give their thoughts on the former's move to Düsseldorf
Holthaus spoke of his two 'very instructive years' in Bochum's first-team, before saying that the transfer was the 'next logical step'.
He cited that fact that Fortuna are a 'family club' and hinted that 'simple and honest work' are the key to success at the ESPRIT arena.
Frank Kramer, who had recently been appointed as head coach, was similarly happy:
"Fabian Holthaus is a young player with great potential," he began. Kramer then cited his experience at a young age as something that, 'says a lot about his talent'.
Kramer also added that his 'aggressiveness in a tackle' and 'tireless running' are two qualities needed for 'a modern full-back'.
Adbat and Holthaus gone, Perthel the only left-back remaining
After Holthaus' departure, it's expected that Nicolas Abdat, another young left-back, will follow in his footsteps - VfL Wolfsburg are rumoured to be interested.
That leaves Timo Perthel as the club's only recognised left-back and his performances have been largely average.
Gertjan Verbeek may well dip into the transfer market with the funds available, Ehsan Hajsafi has emerged as an early front-runner.
Out of contract this summer, the 25-year-old has bundles of experience at international level for Iran and has hinted at a move to Europe.
Hajsafi can also play at left-wing and in the holding midfield role, making him a valuable asset to the Dutchman's plans.