Hartley arrived at Dens Park today for training and took the morning session as usual.
But, it is understood that a board meeting has taken place at the same time, and that has resulted in today’s announcement.
A statement from the club said: “The club currently sit 11th in the Premiership table and the board of directors feel a change is needed ahead of five vital matches between now and the end of the season.
“Managing director John Nelms is currently addressing the players and staff and the club will release a further update shortly.”
Nelms added: “I want to thank Paul for his time with the club. Unfortunately, the business we are in, sometimes change is necessary to achieve the goals we have set.
“I think somebody in the near future is going to get a good hard working manager.”
Dundee have dropped into the play-off position after seven defeats in a row.
They have five games to stay in the top flight, starting with Motherwell a week on Saturday, and a new manager will be in charge for them.