CBS Sports' Jon Heyman.
The deal is reportedly for three years and $18 million.
Affeldt did not give up a run in 10.1 innings of relief in the entire 2012 postseason on the Giants' way to a World Series title.
Affeldt is just one of the Giants' three key free agents this offseason. Marco Scutaro and Angel Pagan are expected to be a little harder to sign than Affeldt, considering Scutaro's outstanding end to the regular season and stellar postseason. Pagan is least likely to re-sign with the Giants, as he is expected to have several teams lining up for a speedy lead-off hitter that can play center field and hit for moderate power.