"Big news," Martin wrote on his Twitter account. "Beyond blessed."
In another Tweet, he wrote: "Opportunities are few in the NFL... Can't wait to get to work #9erEmpire."
The 49ers had opened free agency with a sudden swap of safeties, adding safety Antoine Bethea, a two-time Pro Bowler from the Indianapolis Colts, after Donte Whitner left to join his hometown Cleveland Browns.
In other moves, the 49ers re-signed kicker Phil Dawson (two years) and cornerback Eric Wright (one year) and made another surprising trade: getting quarterback Blaine Gabbert in a deal with Jacksonville. Gabbert, the 10th overall pick in the 2011 draft, has been such a disappointment that the Jaguars parted company with him for a sixth-round pick and the possibility of more compensation depending on Gabbert's role with the 49ers in 2014.
That is not expected to be much, at least in the view of Bill Polian, the former Indianapolis Colts executive who now serves as an analyst for ESPN.
"I would not count on him to contribute to the squad," Polian said of Gabbert.
The secondary's overhaul could have the biggest impact, however. Part of that transition was cornerback Carlos Rogers' release after three seasons to help ease salary-cap constraints.
With cornerback Tarell Brown joining Rogers in free agency, gone are all four starting defensive backs from the 2012 season's Super Bowl run. Safety Dashon Goldson started that exodus a year ago when he grabbed the Tampa Bay Buccaneers' loot, and Whitner followed suit Tuesday with a four-year, $28 million deal to help restore the Browns.
"My time in San Fran will always be cherished," Whitner posted on Twitter. "I love everything about the place. I left my heart and soul out there each and every week."
Bethea thus becomes the favorite to pair with second-year safety Eric Reid. Bethea signed a four-year deal that's worth $4 million to $5 million annually, according to various reports.
"Glad to be a Niner!! Going to the Bay!!!" Bethea posted on Twitter.
Bethea, 29, is a year older than Whitner, but both came out of the 2006 draft. Bethea was a sixth-round pick out of Howard whereas Whitner went No. 6 overall out of Ohio State (to the Buffalo Bills).
Unlike Whitner, Bethea rarely draws penalties and went all of last season without committing one, as was the case with Reid's rookie year. Meanwhile, Whitner got flagged five times for unnecessary roughness last season, including in the NFC Championship game loss to the Seattle Seahawks.
"The business side to the league is never fun," Reid posted on Twitter. "Hate to see him go but happy for my man (Whitner). Appreciate everything you did for me."
Joining Reid and Bethea in the starting secondary likely will be cornerbacks Chris Culliver (out with a knee injury in 2013), Tramaine Brock (five interceptions in seven starts last year) and Wright, who figures to replace Rogers as the slot corner. More cornerbacks could be coming, though the 49ers don't have much salary-cap room to make a play for the Buccaneers' Darrelle Revis.