Six financial institutions were fined €1.71 billion ($2.32 billion) by European Union regulators Wednesday for colluding in an attempt to manipulate key benchmark interest rates, the EU's largest-ever penalty in a cartel case.

The settlements involved penalties against some of the world's biggest banks, including Deutsche Bank AG, Société Générale SA, Royal Bank of Scotland Group PLC J.P. Morgan Chase & Co. and Citigroup Inc. The action brings to roughly €6 billion the total penalties levied by regulators against financial...