Coach Hue Jackson paid a high price to acquire a quarterback he knows well but who has struggled in recent years and refused to report the Bengals this season despite being under contract through 2014.
The Bengals had been adamant about not trading Palmer, who wanted to be dealt from a team that has had only two winning records in the last 20 years.
Owner Mike Brown repeatedly insisted he wouldn't consider Palmer's request for a trade because he didn't want to reward him for holding out. He changed his mind after the Raiders offered a 2012 first-round pick and a second-rounder in 2013 for the 31-year-old quarterback.
The Raiders (4-2) became desperate for a quarterback after Campbell broke his collarbone during a win over the Browns on Sunday. Campbell had surgery Monday and was expected to miss at least six weeks, leaving the Raiders with only Kyle Boller and Terrelle Pryor on the roster.
Jackson's mantra all season has been "the time is now," and he backed that up by dealing for Palmer, who is coming off a 20-interception season last year with the Bengals.
Brown said the play of rookie quarterback Andy Dalton made it easier to trade Palmer.
"We also find ourselves rather suddenly in position of being able to receive real value for Carson that can measurably improve our team, which is performing well and is showing real promise for this year and years to come," he said in a statement. "When this opportunity arose, we felt we could not let it pass and needed to take a step forward with the football team if we could."
Palmer, who had been working out in Southern California, already reported to the Raiders' facility and will immediately start learning the offense. Oakland hosts Kansas City on Sunday and then has a bye week.
While Palmer has not played or practiced since last season, he has a history with Jackson, who was his offensive coordinator for two years at USC and the wide receivers coach for three seasons in Cincinnati.
Jackson was with the Bengals when Palmer had his best season in 2005, when he threw for 3,836 yards with 32 touchdown passes and a 101.1 passer rating while leading the team to an AFC North title. Palmer tore up his left knee during a playoff loss to Pittsburgh that season.
He came back and had two solid seasons before partially tearing a ligament and tendon in his passing elbow during the 2008 season. He has not been an elite quarterback since, despite getting back to the playoffs in 2009.
Over the past two years, Palmer completed 61.2 percent of his passes for 7,064 yards, 47 touchdowns, 33 interceptions and a passer rating of 82.9 while posting a 14-18 record. Those numbers are comparable to what Campbell has done since the start of the 2009 season.
But the Raiders were not willing to trust their playoff chances with Boller, who had not started a game since 2009 and had lost his previous 10 starts since October 2007, or Pryor, a project who will need time before he can be an NFL quarterback.
"It'll be a learning curve for him because he hasn't played football in a while but I'm excited to have a leader on that side of the ball of his caliber," Raiders defensive tackle Richard Seymour told SiriusXM NFL Radio. "Anytime you have an opportunity to acquire someone of Carson's pedigree I don't think it's something that you can pass up on."
This is the second trade the Raiders have made since the death of longtime owner Al Davis, who also served as general manager and oversaw the entire football operation. Jackson dealt last week for former No. 4 overall pick in 2009, linebacker Aaron Curry from Seattle.
The trade leaves the Raiders with picks only in the fifth and sixth round in next year's draft. They traded their second-rounder during April's draft to New England for the picks to draft offensive lineman Joe Barksdale and running back Taiwan Jones. They used their third-rounder to take Pryor in the supplemental draft in August. They traded their fourth-rounder in 2010 to get Campbell and the seventh-rounder for Curry.
Oakland is expecting to get compensatory picks after losing Nnamdi Asomugha, Zach Miller, Robert Gallery, Thomas Howard and Bruce Gradkowski in free agency.
The Bengals (4-2) have started well with Dalton taking Palmer's place. The message board by the entrance to the Bengals' locker room Tuesday had an anonymous scrawled message: "Let My People Goooooo!" Otherwise, there wasn't much reaction from a team that had moved on from Palmer a long time ago.
"I don't think even one player in this locker room's even thought about that," left tackle Andrew Whitworth said. "We haven't worried about it. We've gone forward with the guys we have and tried to play as good as we can and that's all we can do."
The Bengals severed ties with Palmer when the season started and he didn't show up, giving his locker to Dalton, a second-round draft pick.
As recently as Monday afternoon, coach Marvin Lewis reiterated there was no change in the team's position regarding Palmer. Then came the offer from the Raiders.