Siewert arrives from Borussia Dortmund II, the former club of David Wagner, with Town chairman Dean Hoyle hopeful that the German can replicate the success of his predecessor in West Yorkshire.
Siewart has coached Dortmund’s under-23s side since July 2017 after succeeding Daniel Farke, following his move to Norwich City.
On Siewert, whose contract runs until the summer of 2021, Hoyle said: “I am tremendously excited about working with Jan as our new head coach.
“Let me start by addressing the obvious. We enjoyed tremendous success under our previous head coach David Wagner, and we have subsequently appointed a new head coach that bares many similarities to him; a young, aspirational German from Borussia Dortmund II.
“However, that does a disservice to Jan, who is his own man. There is much more to this appointment than that.
“Like any good football club – or successful business outside football – we have succession plans in place for our key roles. A major one here is the role of head coach.
“Given David’s success at Huddersfield Town, we knew that it was prudent to look to the future in case an offer came in that he could not refuse. David was fully aware that we were undertaking this work; you must prepare for the future.
“When David let us know he intended to step aside at the end of the season, we quickly moved to meet Jan; his name was still at the top of our list of possible successors.
“There was an immediate connection; you could see why he was so highly thought of. He is articulate, passionate and he has a huge ambition to be a success”
Following a playing career in Germany with TuS Mayen, SG Bad Breisig and TuS Montabaur, Siedert moved into coaching and has gained experience as a coach in Germany’s international youth sides as well as senior management roles at VfL Bochum, Rot-Weiss Essen and his role as Dortmund.
Hoyle added: “Jan’s name came to our attention in his previous role as assistant manager and under-19 Coach at VfL Bochum.
“We first spoke with him over two years’ ago and we have kept in communication since, watching his progress with interest.
“His reputation in the game is very good; he is known as a coach who is ambitious, has many qualities and who has strong philosophies.
“As such, it came as no surprise to us when he moved to one of the world’s biggest clubs; Borussia Dortmund.
“Jan believes in this club and what can be achieved here. He has turned down several approaches to leave Borussia Dortmund in the past, but he jumped at the opportunity to join us.
“He believes in our ethos and likes what he knows about Huddersfield Town, which made me very proud.”
Siewert’s first match in charge will be at home to Everton on Tuesday, January 29.