Tuesday's match was the German's 100th match in charge but was the latest in a string of disappointing results this season, with Chelsea having suffered two Premier League defeats in their first six matches too.
New owners Todd Boehly and Behdad Eghbali have worked hard all summer to impose their authority at Stamford Bridge, but sacking a manager who guided the club to the Champions League title 15 months ago represents the boldest and most sweeping change yet.
Chelsea said in a statement: "On behalf of everyone at Chelsea FC, the Club would like to place on record its gratitude to Thomas and his staff for all their efforts during their time with the Club. Thomas will rightly have a place in Chelsea’s history after winning the Champions League, the Super Cup and Club World Cup in his time here.
"As the new ownership group reaches 100 days since taking over the Club, and as it continues its hard work to take the club forward, the new owners believe it is the right time to make this transition," the statement continues.
It added there will be no further comment on the matter until a new head coach appointment is made.
Dinamo Zagreb v Chelsea ratings
Tuchel is the second Premier League manager to be fired this season — Bournemouth sacked Scott Parker last week — but news of the German's departure will still have come as a shock to most Chelsea fans despite an indifferent start to the season.
The consortium led by Boehly and Eghbali paid a world-record sports franchise £4.25 billion to buy Chelsea from Russian billionaire Roman Abramovich at the end of May.
The ambitious US magnates are in the process of overhauling every aspect of the Blues’ operation, and have acted quickly to unseat Tuchel.
The highly-regarded German coach has overseen three defeats in seven matches in the new campaign, after the Blues invested a one-window Premier League record £273 million to revamp the squad, recruiting the likes of Raheem Sterling, Kalidou Koulibaly and Pierre-Emerick Aubameyang.
Tuchel played a key role in Chelsea’s recruitment this summer, with Boehly acting as interim sporting director as well as chairman.
The former Paris Saint-Germain coach also steered Chelsea through the mire of UK Government sanctions from March to May last season, amid the wider impact of Russia’s war in Ukraine.
Tuchel arrived in London in January 2021 after the club parted ways with Frank Lampard.
In his first season, the former Borussia Dortmund manager took Chelsea to the FA Cup final, where they were beaten by Leicester City.
Disappointment quickly turned to joy though as Tuchel guided Chelsea to the Champions League title, Kai Havertz's goal clinching a 1-0 win over fellow English club Manchester City in the final.
Tuchel was rewarded with a contract extension to 2024 and the Blues improved to a third-place finish last season. They were knocked out of the Champions League by Real Madrid in the Champions League quarter-final and lost to Liverpool in the FA Cup final.
The Blues’ search for a new sporting director continues, with former Liverpool supremo Michael Edwards still the long-term target.
Now the Blues need a new manager at the Stamford Bridge helm, with Brighton boss Graham Potter and former PSG and Tottenham coach Mauricio Pochettino among the early candidates.
Chelsea take on Premier League rivals Fulham at Stamford Bridge on Saturday.