Siewert was appointed as a replacement for David Wagner in January but he could not prevent their relegation from the Premier League as they finished bottom of the pile.
Huddersfield Town have taken just a point from their three Championship games so far and Friday night’s 2-1 home loss to Fulham proved to be Siewert’s last game in charge.
In all, the 36-year-old German managed 19 games but won just once; a 1-0 victory over Wolves in February and suffered 15 defeats.
On Saturday, Terriers chairman Phil Hodgkinson said: "Last night's decision was not one that I nor the board took lightly, but I believe it is one that had to be made.
"Jan walked into a very difficult situation in January; something that we were all aware and mindful of. However, I have seen no evidence of the impact on our performances that I was expecting following a full pre-season.
"Jan is a fantastic man and he's worked tirelessly for this club during his time here, but unfortunately it has not worked. I believe he is a good coach and I sincerely hope he will be successful in the future.
"I believe our players are better than we have seen so far this season. We will work to appoint a new head coach who can get the best from them and bring an identity back to our performances."
First team coach Mark Hudson has been placed in temporary charge until a new manager is hired.
