The German had been in charge of Borussia Dortmund's Under-23 team since July 2017 before joining the Terriers.
Following a playing career in Germany with TuS Mayen, SG Bad Breisig and TuS Montabaur, he moved into coaching.
Siewert started his coaching career with Rot-Weiss Essen in the German fourth tier in 2015, while also working as a coach for Germany's Under-17 and Under-18 sides.
He moved to Dortmund after a spell as assistant manager at Bochum.
18 months later, he was confirmed as head coach of Huddersfield Town.