The stock plummeted 15.13 percent to €1.15 on the Frankfurt stock
exchange Tuesday, by far the worst performance among the DAX 30 companies, whose index was down 1.22 percent to 5.537,39 points.
Commerzbank is one of the German banks most heavily exposed to Greek debt.
Last month the European Union's financial industry watchdog, the European Banking Authority (EBA), said the bank would need additional capital of €2.9 billion ($3.9 billion) by mid-2012 – nearly 60 percent of the core equity capital needs of the entire German banking industry.
On Tuesday, rumours circulated that Commerzbank's needs may now have swelled to around €5 billion, sparking a sell-off of the bank's shares.
A company spokesman declined to comment.
Commerzbank, which turned to the German government for help during the 2008 financial crisis, said this month it was abandoning its profit target for next year after writedowns on its Greek bond holdings pushed it deeply into the red in the third quarter.
It added it was examining all options to meet the EBA's requirements including speeding up the sale of non-strategic assets and temporarily suspending all new loan business outside its core regions of Germany and Poland.