Citigroup Inc., Royal Bank of Scotland Group Plc and JPMorgan Chase & Co. are among five banks that agreed to pay European Union fines totaling 1.07 billion euros ($1.2 billion) for colluding on foreign-exchange trading strategies.

Citigroup was hit hardest with a 310.8 million-euro penalty, followed by fines of 249.2 million euros and 228.8 million euros for RBS and JPMorgan, the European Commission said in a statement on May 16. Barclays Plc was fined 210.3 million euros and Mitsubishi UFJ...