Votto, who was batting .342 with 14 home runs and 49 RBIs, will be placed on the 15-day disabled list.
"Joey Votto met with the medical staff this evening and has been diagnosed with a torn meniscus in his left knee," Reds General Manager Walt Jocketty said. "He is going to have arthroscopic surgery tomorrow [Tuesday] and should miss 3-4 weeks. Joey is very comfortable with the decision and is eager to return to the lineup."
Votto, named to his first All-Star Game this month, has been slumping recently, going 9-for-32 since July 3.
"It is in my best interest and in the best interest of the team to do it now, so that I can be healthy during the last two months of the pennant race," Votto said.