Petrobras plans to sell up to $US64.5 billion worth of new stock, in one of the world's largest public share offerings.
The Brazilian state oil company said the transaction could be expanded to as much as $US74.7 billion if there were heavy demand.
The money will fund the development of recently-discovered oil reserves off the coast of Rio de Janeiro.
Shares in Petrobras rose 4% in late afternoon trade in Sao Paulo on Friday.
The company said it would issue 2.17 billion common shares and 1.58 billionn preferred shares.
The price of the new shares will be announced on 23 September.
A public share offer had been expected earlier this year but was delayed while a deal was sorted out over how many shares the Brazilian government would receive in return for giving Petrobras access to up to five billion barrels of oil.
A price of $US8.51 per barrel was eventually agreed.