Shares of Citizens Financial Group, the U.S. unit of Royal Bank of Scotland, rose less than 1 percent in their debut, valuing the company at about $12 billion in the biggest U.S. offering by a bank since the financial crisis.
The offering, which raised $3.01 billion for RBS, is the second biggest in the United States this year after Alibaba Group Holding's $25 billion IPO last week.
Citizens Financial's IPO was priced at $21.50, below the expected range of $23-$25.
RBS owned 100 percent of Citizens Financial before the offering.
Citizens Financial's shares were at $21.64 shortly after the start of trading.
Citizens initially said it expected its IPO to be priced between $23 and $25 per share. RBS is selling all 140 million shares in the offering and could sell a further 21 million in an over-allotment option granted to the underwriters of the offering.
RBS, which is 80 percent-owned by the British government, is hiving off the 186-year-old Rhode Island-based bank as it is under pressure from regulators and lawmakers to bolster its capital and focus on lending to UK households and businesses.