Singletary originally indicated he would probably wait until after his team's third pre-season game to decide his starter at quarterback, but needed only two contests to make up his mind.
"I'm very excited about how Shaun ended last season and how he's progressed through the off-season," Singletary said.
Hill dueled with Alex Smith, the No. 1 choice in the NFL draft in 2005 who missed all of last season with a shoulder injury. Hill was the starter for the final seven games in 2008 and the 49ers closed the season with five victories in six contests.
"I brought them in together and had them understand why we made the decision and how important it is for them to continue to push each other and compete," Singletary said. "I thanked them for how they've been throughout this whole process. I feel very fortunate to have both of them. They both made great strides."
San Francisco will open its season at Arizona, the defending NFC champion.