The 37-year-old German managed just one victory from his 19 games in charge.
Siewert took over in January after David Wagner left the club, but could not prevent their relegation from the Premier League.
His only win was a 1-0 victory over Wolves in his fifth game at the helm as Huddersfield finished bottom, relegated by March.
A club statement read: “Huddersfield Town has this evening terminated the contract of head coach Jan Siewert with immediate effect.”
Huddersfield’s poor form continued into this season, with the club having picked up just one point from their opening three Championship games.
They were also knocked out of the Carabao Cup at home to League One Lincoln.
Town’s statement added: “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.”
Ivan Cavaleiro’s stunning curler for Fulham 10 minutes from time proved the final straw for the Huddersfield hierarchy after Karlan Grant’s third goal of the season had cancelled out Aleksandar Mitrovic’s opener.
Fulham boss Scott Parker said: “We are getting to grips with the league and we are learning what it is like as quick as we can.
“This is a tough league to play in and we got a wake up call against Barnsley on the opening day,” he said.
“We have been superb since Barnsley. We understand what life is about and the last two results have been fantastic.”
Parker continued: “Huddersfield are struggling a bit at the moment but we knew it would still be a tough place to come.
“I thought we rushed and forced things when we did not really need to do in the first half.
“We spoke at half-time about controlling the game and getting a platform to allow us to do that.
“We did that in the second half. It was a great goal to win it with and a pleasing performance.”