Six-time All-Star Beltran, the most sought-after hitter on the market in advance of Sunday's trade deadline, should provide a boost for the World Series champion Giants.
"Our offense has been inconsistent and we need some help," Giants manager Bruce Bochy told Major League Baseball's website (www.mlb.com). "There was a little sense of urgency to see if we could do something to help this offense get going."
San Francisco gave up top pitching prospect Zack Wheeler in the deal, which has not been officially announced by either club.
The hard-throwing, 21-year-old Wheeler was 7-5 for Single-A San Jose this season with 98 strikeouts in 88 innings.
Hobbled by injuries the previous two seasons, Beltran has been healthy and productive this year for the Mets.
The 34-year-old Beltran was leading Major League Baseball in doubles with 30 while hitting .289 with 15 home runs and 66 runs batted in.
Beltran, who has played the past seven seasons for the Mets, was due to be a free agent next year and unlikely to be signed to another long-term deal by the New York team.
The Mets are expected to pick up $4 million of the remaining $6.5 million on his contract.
(Reporting by Gene Cherry in Raleigh, North Carolina; Editing by Larry Fine)