The 50-year-old coach has handed in his resignation as he feels the club lack the necessary conditions for him to do a good job, admitting he regrets joining the team
The former Hungary boss was in his first season at Utrecht, who are 10th in the league with 12 points from nine games.
Koeman had a frosty relationship with the club's board, especially with technical director Foeke Booy, following the signing of Rodney Sneijder from Ajax without his approval and the departures of several key players in the summer.
"The working situation at the club was quite poor," the 50-year-old told reporters. "It will never be perfect and I am not a demanding person.
"However, the basics were not even there. I need to focus on my job for the full 100 per cent. I have not had much sleep lately.
"Eventual changes in the future would not help me as I only had a contract until 2012. Utrecht are not the right club for me. I made a wrong decision."
Jan Wouters, one of Koeman's assistants, has been named as Utrecht's new coach as the club prepare for the weekend match against Heerenveen.