McDermott guided the Royals into the Barclays Premier League for only the second time in their history last season but they have struggled for form and now sit 19th, with only goals scored keeping them ahead of bottom side QPR.
Zingarevich had consistently backed McDermott in public and the former Arsenal midfielder collected the Barclays Premier League manager of the month award in January after a run of one defeat in seven games for the Berkshire club.
Reading released a statement on Monday afternoon announcing the decision and said the process to appoint a replacement was already under way.
"Brian gained promotion to the Barclays Premier League last year for only the second time in the club's history thanks to a remarkable run at the end of last season," the statement read.
"However, in our current situation, owner Anton Zingarevich felt that a change was necessary.
"The search for a successor will start immediately and it is hoped that a new manager will be in place as soon as possible to lead the fight against potential relegation."
The club later announced that Academy head Eamonn Dolan would take charge of first-team matters going into the upcoming match against Manchester United.