Manager Bruce Bochy made the move because he felt having a more patient hitter in that spot would help jump-start the Giants' offense, anemic of late.
Entering Saturday's matchup with the D-backs, the Giants had scored six combined runs in their previous four games.
"He's a pretty disciplined hitter, so I think it'll be good," Bochy said. "I think any time you try to get some rallies going, you want to get someone who has good discipline, who can get you a walk and get on base."
Belt ranks second on the team in walks with 10, however, the 24-year-old boasts that number in only 60 at-bats.
Melky Cabrera, who leads the Giants in walks with 12, has 131 at-bats.
Belt is batting just .250, but with the help of the base on balls, his on-base percentage is .357.
"Table setters, we've talked about putting Belt in there for a while," Bochy said. "I want to see how he goes."