NFL Media Insider Ian Rapoport reported Monday that the deal will pay $12 million over two years, per a source who had seen the deal.
"Looking forward to joining my teammates, coaching staff and fans to bring back another championship to the Bay Area," Boldin wrote. "Let's go after our Quest For Six."
Boldin had 85 receptions for 1,179 yards and seven touchdowns in 2013, his first season in San Francisco.
The 33-year-old has aged incredibly well because he never was reliant on speed. He is a big-bodied receiver who can dominate a game with route running, positioning and attacking the ball.
Boldin is the type of player who immediately opens up a defense for the rest of the players in his offense -- both receivers and running backs. It's no surprise that offenses in Baltimore and Arizona have struggled to replace his presence.