The team didn't disclose terms of the deal, but the former Denver Broncos cornerback told ESPN's Josina Anderson on Tuesday that he had agreed to a one-year deal.
"I am going to Oakland," Porter said in a text message.
"They presented the best opportunity for me to get my career back on track after what transpired last season with the Denver Broncos," he said in another text.
A source told ESPN.com that Porter's deal is worth $2.5 million.
Porter is likely to fill one of the Raiders' two starting cornerback vacancies. The Raiders also recently attempted to sign Terence Newman, but the cornerback decided to re-sign with the Cincinnati Bengals and tweeted last week that he turned down more money from the Raiders to accept Cincinnati's offer.
Oakland's interest in Porter is not surprising, as coach Dennis Allen was his position coach in New Orleans. Porter left the Saints to sign a one-year deal with Denver last offseason.
He started the season as a starter but he fell out of favor with the team after he dealt with the effects of a seizure suffered in the summer. He never regained playing time once he was healthy.
Porter has eight career regular-season interceptions, plus two in the postseason, including a pick of Peyton Manning in Super Bowl XLIV that he returned 74 yards for a touchdown to seal the Saints' 31-17 victory over the Indianapolis Colts.
ESPN's Josina Anderson and ESPN.com AFC West blogger Bill Williamson contributed to this report.