Siewert replaced David Wagner in January 2019 but was unable to help them escape relegation from the Premier league at the end of the season.
Hopes of an immediate return to England's top flight were already looking doomed, with Huddersfield picking up just one point from their opening three games.
Huddersfield Town chairman Phil Hodgkinson has explained why the Terriers axed boss Jan Siewert.
Huddersfield's decision came down Friday night, just minutes after the club suffered a 2-1 home defeat to fellow relegated side Fulham.
'Jan walked into a very hard situation in January; something that we were all aware, and mindful, of.
Staring down both barrels of a relegation gun, Town went German again with his replacement Jan Siewert, again from Dortmund.
'However, I have seen no evidence of the impact on our performances that I was expecting following a full pre-season.
Siewert was nearly completely unknown to English football followers when he took over at Huddersfield after Wagner left in January. If they bring the right person in, this can still be a good season, we're only three games in.
Their poor form continued into the new Championship season, losing at home to Derby County before picking up a draw away at Queens Park Rangers.