The contract is worth $3 million, sources told ESPN’s Adam Schefter.
Del Rio said it is “not out of the question” that Bowman could play on Thursday night against the Kansas City Chiefs.
Bowman fills a need for the Raiders at middle linebacker with rookie fifth-round draft pick Marquel Lee out with an ankle injury.
“He’s a veteran guy. He’s played a lot of good ball over the years,” Del Rio said. “We’re excited to have him. We’re a little green at the linebacker position. To add somebody with the experience that he has, we think it’s a nice addition for us and we’ll get him up to speed and we’ll see how quickly we can get him on the field.”
The 49ers released Bowman on Friday after seven seasons with the team.
Before that move, the 49ers had agreed to trade the veteran to the New Orleans Saints for a seventh-round draft pick, league sources told ESPN. But after agreeing to the trade, the 49ers learned that Bowman preferred to be a free agent and decide where he would play next.
As a reward for his service, San Francisco acquiesced and released the linebacker.
In his time with the 49ers, Bowman became one of four players in franchise history to have four first-team, All-Pro nods and finished his time in the Bay Area with 709 total tackles, 12 1/2 sacks, 7 forced fumbles and 4 interceptions.
Bowman also provided some of the most memorable moments in franchise history, including his 89-yard interception return for a touchdown that nailed down a playoff spot in 2013 and closed down Candlestick Park, and the pass breakup in the NFC Championship Game that sent the Niners to Super Bowl XLVII.
All of that despite a knee injury that kept him out for the entire 2014 season and an Achilles injury that cost him the final 12 games of last season.
Through the first five games of the season, Bowman had 38 tackles, but he’d also played 333 snaps, second-most among linebackers.
In his final game for the 49ers, against the Indianapolis Colts in Week 5, Bowman played 58 of 76 defensive plays, well short of his usual full workload in which he rarely comes off the field.
After that game, Bowman voiced his frustration to reporters, acknowledging that he did not like the decision to get him extra rest. He repeatedly said “I don’t know” when asked if he at least understood the rationale behind it.
ESPN’s Paul Gutierrez and Nick Wagoner contributed to this report.