Unibanco is the Brazilian partner of Mr. Gates' U.S. software company, Microsoft Corp.
Mr. Gates is cleverly slotted into a long-running humorous campaign by W/Brasil, Sao Paulo, in which couples compete to be chosen as the "Unibanco Couple" and appear in commercials.
Washington Olivetto, president of W/Brasil, said he mentioned to Unibanco that he would love to work with Mr. Gates, who agreed immediately when Unibanco executives approached him informally.
In the spot, shot late in April in the U.S., aspiring Unibanco couple Luiz Andre and Renata are at home glued to their computer and TV set respectively as they await word from Unibanco. When Bill Gates appears on the TV screen using the same software program Luiz Andre is absorbed in, Renata calls Luiz Andre over.
Expecting to see himself on the screen as the new Unibanco spokesman, Luiz Andre is distressed instead to see Mr. Gates.
Mr. Gates then says in English: "Unibanco and Microsoft are working together in a home-banking system that's really cool. You should have one." Subtitles appear in Portuguese.
Luiz Andre exclaims, "That's Bill Gates! The owner of Microsoft!"
Still on TV, Mr. Gates says jovially, "I'd like to say hi to the Unibanco couple, Renata and Luiz Andre. Hi!"
The spot ends with the dumbstruck couple's jaws dropping as the Unibanco logo appears on screen.
"He's practically an actor," Mr. Olivetto said enthusiastically of Mr. Gates.
The U.S. computer magnate, who did the spot for free to promote Microsoft in Brazil, is no acting novice. In the U.S., he appears in an ad for Nasdaq, the stock exchange Microsoft is listed on, as part of Nasdaq's campaign to show off the high-profile companies like Microsoft that make their home on that exchange.