Boldin, who was entering the final year of his contract, agreed to a three-year, $25 million extension with the Ravens, ESPN NFL Insider Adam Schefter reported. He has four years, $28 million left on his contract. The extension includes $10 million in guaranteed money.
"The Ravens just got better," coach John Harbaugh said. "Anquan is a significant addition for us. He fits the personality of our team with the hard-nosed, physical way he plays. We love the way he competes. Our fans will enjoy watching him compete and his teammates will be excited to have him with us."
The Ravens also received Arizona's fifth-round draft pick in 2010.
Baltimore's top priority this offseason was to bolster a wide receiving corps that last season featured Derrick Mason, Mark Clayton, Kelley Washington and Demetrius Williams. Mason was the lone receiver with 1,000 yards, and running back Ray Rice led the team with 78 receptions.
Mason and Washington are unrestricted free agents, but with the addition of Boldin and free agent Donte Stallworth, who was signed last month, Baltimore appears to have improved its wideout situation regardless of whether Mason and Washington return.
The 6-foot-1, 217-pound Boldin was the Cardinals' No. 2 receiver behind Larry Fitzgerald. Boldin is a physical player capable of making a crushing block or a difficult catch, but his one shortcoming has been injuries. He has had one injury-free season since 2003 and last season missed both of Arizona's playoff games with ankle and knee injuries.
But the Ravens couldn't be happier to have him.
"Anquan is a player who makes the tough catches and he is outstanding getting yards after catches," Ravens general manager Ozzie Newsome said. "He will also help the running game with his blocking skills. He's a very tough competitor and, with the way he plays with passion, he will fit right in with the foundation we have for our team."
Boldin was delighted with the news.
"I'm definitely excited. For me, I've been hoping for this for a year since I first heard that the Ravens might be interested in me," he said. "The Ravens play a certain way. They play as hard and as physical as any team in the league, and I want to add to that. I think I play the way they play."
The trade of Boldin has grown ever more likely since the opening of Arizona's training camp in 2008, when he accused the Cardinals' management of lying to him by promising a new contract during that offseason.
He went on to demand a trade, but the Cardinals didn't find anything to their liking a year ago.
The Ravens were always the front-runner, Arizona general manager Rod Graves said.
"We did have several teams approach us over the weekend and express interest during the combine," Graves said, "but Baltimore certainly was much further ahead from a contractual standpoint with Anquan and his agent and the compensation we felt was fair for us. We felt the timing of it was good."
Arizona wasn't prepared to offer Boldin the kind of contract he wanted because of his age and injury concerns. He switched agents and kept quiet about a trade demand last season but said his opinion about the situation had not changed.
Cardinals coach Ken Whisenhunt said it was critical Boldin get a new contract as part of any trade.
"Mutually for both parties it was a good deal," Whisenhunt said. "I am sad to see Anquan go just because of the great respect I have for the way he played the game, certainly for the contributions that he's made to our ball club since I've been here. But I am also pleased that Anquan gets something that he's excited about, something that he's worked hard to get."
Boldin never let his contract situation affect his effort. He remained a team leader in the Arizona locker room was one of the Cardinals' toughest and hardest-working players.
Boldin holds the Cardinals franchise record for receptions at 586. His 7,520 yards receiving rank third and his 44 touchdowns rank fifth in team history.
When Boldin went on his training camp tirade against the management in training camp two seasons ago, he said he would never re-sign with Arizona. He did, however, seem to soften that stance somewhat after replacing agent Drew Rosenhaus with Tom Condon.
The Associated Press contributed to this story.