A meeting of the club board voted to part company with Tudor, who had only joined the Istanbul giants in February, Galatasaray said in a statement.
Galatasaray are hungry for success having seen Istanbul arch rivals Besiktas claim the title in each of the last two years.
The Croatian had been under huge pressure to deliver since arriving in Istanbul and last weekend’s 2-1 defeat at little-heralded Yeni Malatyaspor marked the final straw.
The defeat saw Galatasaray cede top spot to fast-rising Basaksehir, also from Istanbul, with Besiktas sniffing at their heels in third.
Turkish Super Lig clubs are notorious for sacking coaches but Tudor’s dismissal — with the team second and one point behind the leaders — is especially brutal.
The situation at Galatasaray has been extreme, with Tudor the sixth boss to take charge of the club since 2014.
His position had been under severe pressure due to poor derby results, including 5-1 and 3-0 thrashings at the hands of Basaksehir and Besiktas respectively.
Speculation will now be intense over his successor, with Turkish media pointing to a possible return for icon Fatih Terim, the former Turkish national boss who has coached the club on three different stints.
Galatasaray had spent lavishly in the close season, digging deep for the likes of French international striker Bafetimbi Gomis, Morocco midfielder Younes Belhanda, Brazilian midfielder Fernando and Algeria midfielder Sofiane Feghouli.