Siewert, 36, has left his position as coach of Borussia Dortmund's second team to join the Terriers on a deal until summer 2021.

Wagner, who held the same position at Dortmund before joining Huddersfield in 2015, left the club by mutual consent on 14 January.

Huddersfield are bottom of the Premier League on 11 points - 10 adrift of safety - with 15 games remaining.

Siewert's first game in charge will be at home to Everton on 29 January.

The club announced the appointment on Twitter with a video of a Manchester City fan from Wakefield, who was mistaken by TV for Siewert during Sunday's game at the John Smith's Stadium, taking a phone call in the manager's office.

"Not now, Martin from Wakefield," says the German, adding: "Hi, I am Jan. I am really happy to be the new head coach of Huddersfield Town."

Mark Hudson took temporary charge for Sunday's defeat by Manchester City.

Terriers chairman Dean Hoyle said: "We enjoyed tremendous success under our previous head coach, David Wagner, and we've subsequently appointed a new head coach that bears 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."

Hoyle says the club have "succession plans in place for our key roles" and had followed Siewert's career "with interest".

He added: "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 couldn't refuse. David was fully aware that we were undertaking this work - you must prepare for the future.

"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've kept in communication since.

"His reputation in the game is very good. He's 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."

Hoyle also said that Siewert has turned down several approaches to leave Dortmund, but "jumped at the opportunity" to join the Premier League club.

"He believes in our ethos and likes what he knows about Huddersfield Town, which made me very proud," added Hoyle.

"In the short term, we will continue to fight as hard as possible against relegation from the Premier League. We're not giving up on that.

"We will also plan for the medium and long term, whatever the outcome. Jan is committed to us and is excited to take on the challenges we have ahead, whatever they look like."