Postbank, with nearly 15 million customers, is the country's largest retail bank, Deutsche Welle reported Friday.
The deal comes with the option to push the consolidation further. Deutsche Bank has the option to purchase another 18 percent of Postbank within three years, the report said.
It is the second large deal between German banks arranged this month. Earlier, Commerzbank purchased Dresdner Bank from the insurance company Allianz for $13.88 billion.
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