They announced Polanski's departure a day after Saturday's 5-0 collapse to Freiburg that kept the Foals anchored in last place. Polanski, a former longtime youth and senior Gladbach player, took over in September 2025.
"Despite a good pre-season and some encouraging signs, the team haven't been able to get the results we were looking for in the Bundesliga," said Gladbach Head of Sports Rouven Schroder in a statement.
"Our recent performances ultimately led us to the conclusion that a change was needed, and we have therefore relieved Eugen of his duties with immediate effect."
Gladbach lost their season opener against RB Leipzig 3-0 before conceding three second-half goals to lose 4-3 to promoted Elversberg a week later.
Former Mainz 05 coach Jan-Moritz Lichte will take charge of the team on an interim basis.