Mario Manningham is making the switch from year deal on Thursday.
Manningham, who played for the Giants during the first four years of his career and was a key factor in New York's Super Bowl win over New England this past season, made 39 receptions for 523 yards with four touchdowns during 12 regular-season games in 2011. He missed four contests with a knee injury.
Manningham had three touchdowns over his four playoff games last season, and made a huge 38-yard reception along the sideline during New York's winning drive to beat the Patriots.
"We are very pleased to add a player like Mario to our team," said 49ers general manager Trent Baalke. "Although it is still early in his NFL career, he is a productive wide receiver with a lot of big game experience. He will be a nice compliment to our current wide receiver group and a good fit for our offensive system."
Originally drafted by the Giants in the third round of the 2008 NFL Draft, Manningham has recorded 160 receptions for 2,315 yards and 18 touchdowns in 49 career games (28 starts).