At the heart of the suit is the question of when Deutsche took control of Postbank from its previous owner, logistics firm Deutsche Post.
The investors argue that Deutsche Bank was already in de facto control of Postbank two years before it launched a full takeover of the bank.
"Deutsche Bank intervened significantly in Postbank's business and took over control" as early as 2008, two years before it bought out the plaintiffs' shares, their Munich- based lawyer Oliver Krauss told business weekly WirtschaftsWoche.
Some 31 German and international investors are involved in the suit with claims totalling around 740 million euros.
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According to court documents seen by WirtschaftsWoche, the plaintiffs accuse Deutsche Bank of striking "numerous contracts and agreements, some of them secret" with Postbank's former owner Deutsche Post in 2008.
They allegedly included giving up risky financial market activities in favour of retail banking, details of a capital increase at Postbank, personnel changes in the executive and supervisory boards, and the decision not to pay out a dividend in 2008.
Such agreements helped both sides weather the years after the financial crisis, the plaintiffs argue, saving Deutsche Post from losing money on its investment and offering more flexibility to Deutsche Bank as it battled to right itself.
"We believe the claims made in the suit are unfounded," a Deutsche Bank spokesman told AFP.
A fresh legal suit is unwelcome news for Deutsche, which in recent years has made clearing up old entanglements a priority as part of a wider restructuring.
Shares in Deutsche were largely untroubled by news of the suit today, losing 0.2 per cent to trade at 15.92 euros (USD 19.14), against a DAX index of leading German shares up 0.4 per cent.