Siewert becomes the first managerial casualty in the Championship or the Premier League this season.
The Terriers have lost two of their three matches on their return to the second division, drawing the other.
They were beaten 2-1 at home by Fulham on Friday night and the final whistle was met by a chorus of boos from the home fans.
Karlan Grant had produced a response to Aleksandar Mitrovic’s opening goal after half-time before the on-loan Ivan Cavaleiro curled in a late winner for the visitors.
That means the 36-year-old German coach – who replaced the much-loved David Wagner back in January – leaves after winning just one of 19 matches in charge.
In a statement, Huddersfield did not even thank Siewert and his coaching team’s efforts over the past seven months after cutting short a contract which was due to run until 2021.
They simply said: “Huddersfield Town has this evening terminated the contract of Head Coach Jan Siewert with immediate effect.
“Siewert joined the Club from Borussia Dortmund in January 2019 in the Club’s ultimately unsuccessful fight against relegation from the Premier League and led the team in the first four competitive games of the new season, with his final game in charge being the Sky Bet Championship defeat to Fulham on Friday evening.
“No further comment will be made this evening.”
He was sacked just over an hour after the loss to Fulham.
Former Borussia Dortmund II coach Siewert’s miserable spell at the helm of the Yorkshire outfit also saw Huddersfield stunned by Lincoln City in the Carabao Cup first round this week.
His only win at the John Smith’s Stadium was a 1-0 triumph over Wolves in the Premier League back in February.
Town ultimately finished 20th in the Premier League with a meagre three victories and 16 points from 38 games.
Siewert only managed to win them five points of an available 45 in the top flight but was trusted to continue taking the reins after the drop into the Championship.
But a disappointing start to the new campaign means he has been removed from his role.
First-team coach Mark Hudson is now likely to take over managerial duties until a permanent successor can be found.