Earlier this month it priced the shares in the range of $23 to $25 per share, potentially raising $3.5bn.
The Financial Times reported that some investors were concerned about the outlook for Citizens and were unwilling to pay the higher price.
In a statement RBS chief executive, Ross McEwan said: 'The sale of Citizens is an integral part of the RBS capital plan.
'Selling Citizens will significantly improve our capital position and help us to create a strong and secure bank that can continue to fully support the needs of its customers.'
RBS bought Citizens for $440m in 1988 and subsequently turned it into one of the biggest regional banks in the US.
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