The move came three days after Uggla was released by the Atlanta Braves.
Fox Sports reported Uggla's deal with the Giants earlier Monday.
Giants manager Bruce Bochy said he spoke to Uggla by phone Monday and the veteran is anxious to get started in Fresno.
"This is a chance for us to take a look at him and evaluate him, and a chance for him to go down and get his timing and get some playing time," Bochy told reporters before the Giants-Phillies game at Cirizens Bank Park. "At some point we're going to have to make a call. I can't say he's going to be up here in the next few days, no. He needs some playing time right now."
In his fourth season with the Braves, Uggla, 34, was batting .162 with two home runs and 10 RBIs in 48 games and 130 at-bats.
He had a 33-game hitting streak in 2011, the franchise's second-longest, but also struck out 495 times in 451 games from 2011 to 2013.
Uggla, a three-time NL All-Star, hit .179 with 22 homers and 55 RBIs last year and underwent corrective eye surgery last summer. He struggled so much that he was left off the postseason roster after losing his job to Elliot Johnson, a journeyman claimed off waivers in late August.
The Braves reinserted Uggla as the starter at the beginning of spring training this year, but he hit .212 in April and .080 in 25 at-bats during May.
Marco Scutaro, who came off the disabled list from a back injury on July 12, started at second base for San Francisco in Monday's game in Philadelphia.
Information from ESPN.com's Jerry Crasnick and The Associated Press contributed to this report.