"Borussia Dortmund have released head coach Nuri Sahin with immediate effect following an internal analysis of recent sporting developments," said Dortmund in a statement.
The Ruhr valley club, last year's Champions League finalists, conceded two goals in two minutes in the second half against the Italians after taking a 15th-minute lead.
They have also lost all three league matches in 2025, dropping down to 10th place in the Bundesliga and putting their participation next season in Europe's premier club competition at risk.
"After four defeats in a row and only one win from the last nine games ... we have unfortunately lost faith in being able to achieve our sporting goals in the current constellation," Dortmund managing director Lars Ricken said in the statement.
"This decision also hurts me personally, but it was no longer avoidable after the game in Bologna."
Sahin, 36, leaves the club just a little over half a year after being appointed to replace Edin Terzic as the new coach.
The German-born former Turkey international was a former youth and senior player at the club. He became an assistant coach at Dortmund in 2024 after a two-year coaching spell at Turkey's Antalyaspor. He had signed a contract to 2027.
The club said it would soon announce who would be in charge for this week's league game against Werder Bremen.
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