Banco Santander SA, the Brazilian unit of Spain's Santander, filed on Thursday for an initial public offering in Brazil and the United States, in what could become one of the largest IPOs of the year.

In July, Santander said it would sell about 15 percent of its Brazilian unit through a new share issue, suggesting the IPO would be worth around $5.6 billion based on Banco Santander SA's current market value of around $37 billion. That figure is derived from a tiny number of shares that are currently tradable.

A deal of that size would put it among the top IPOs of 2009 worldwide, and be the largest in the United States since the $19.6 billion offering by credit card operator Visa Inc in March 2008.

The IPO will also be a reflection of the relative strength of the Brazilian banking sector, which has weathered the turmoil in global markets and the credit crunch better than the financial sector in the United States and Europe.

Spain's Santander controls virtually all of the Brazilian unit's common and preferred shares.

Banco Santander SA's filing with the U.S. Securities and Exchange Commission on Thursday indicated it was seeking to raise up to $200 million, but that amount is almost certainly a placeholder until it files an updated prospectus that would include terms such as the number of shares to be sold and a price estimate range.

A spokeswoman for Santander's Brazil unit said the bank would not comment on the transaction because of the quiet period ahead of the offering.

Other major IPOs this year have included a $7.3 billion IPO in July by China State Construction Engineering Corp and a $4.3 billion offering by VisaNet, the credit card operator's Brazilian affiliate.

Banco Santander filed to sell units, each representing 55 common shares and 50 preferred shares, according to the filing. The units will trade on the Sao Paulo stock exchange under the symbol SANB11.SA, the filing said.

The filing did not indicate an about timing for the offering.

Santander is Brazil's third largest private-sector bank and the largest bank controlled by a major global financial group, with a 10.2 percent market share measured by assets as of March 31, 2009, according to its prospectus.

Santander's Brazilian unit had assets of 288.9 billion reais ($154.82 billion) at the end of June.

For the six months ended June 30, 2009, the Brazilian unit's operations accounted for over 20 percent of Santander Group's net income and 53 percent of its net income in Latin America.

Santander has expanded its presence in Latin America's largest economy through a slew of acquisitions in recent years, most recently by taking over ABN Amro's Brazilian unit.

($1=1.866 reais) (Reporting by Elzio Barreto and Phil Wahba; editing by Andre Grenon, Bernard Orr)